Easing fossil-fuel energy regulations is at the top of President-elect Donald Trump‘s to-do list, part of an agenda for his first 100 days that he outlined in a video released Monday.

“I will cancel job-killing restrictions on the production of American energy – including shale energy and clean coal – creating many millions of high-paying jobs,” he said, without saying what kind of coal he considered clean. “That’s what we want, that’s what we’ve been waiting for.”

But getting rid of rules in Washington, D.C. is not as quick as saying, “You’re fired,” in a reality TV show.

To undo a regulation that has been finalized, the Trump administration would have to begin a public notice and comment period all over again, address those comments and publish an analysis to support its decision, regulatory experts said. It is a process that often takes years. The administration would face lawsuits by environmental and public health groups as well as states and industry groups.

And the Trump administration also could face litigation for any failure to regulate. For example, supporters of environmental regulations like the Obama administration’s Clean Power Plan can argue that because the Supreme Court already has ruled that carbon dioxide is a pollutant under the Clean Air Act, the law gives EPA no choice but to act to protect health. (For years, the agency has been under a court order to produce regulations.)

“We’re prepared to use every tool available to protect these environmental safeguards that the public supports,” said David Goldston, director of government affairs for the Natural Resources Defense Council. NRDC has continually pursued such regulatory litigation, both defending rules and suing agencies for failure to act.

“Laws were created to enable these protections to be put in place and defended,” said Goldston. “And we will defend them.”

But some of the Obama administration’s actions to address air and water pollution, land use, and greenhouse gases may be vulnerable. In particular jeopardy are the most recent actions and regulations that already being challenged in court by industry and the states.

Under an obscure law known as the Congressional Review Act, Congress could review and override recent major regulations by a simple majority vote.  In a memo released the day after the election, the Congressional Research Service said that any regulation finalized after May 30 could be subject to Congressional override. (Goldston said that the cutoff date is not entirely clear, and will depend partly on how long the lame-duck session of Congress lasts this year.)  A Congressional override of a rule would have long-lasting consequences; the agency would be prohibited from enacting a similar regulation again in the future.

Congress may take quick aim at the Interior Department’s Methane and Waste Prevention Rule for oil and gas operations on federal lands, finalized on Nov. 15. The Bureau of Land Management issued the regulations to curb venting, flaring and leaks of natural gas to reduce harmful emissions of the potent greenhouse gas methane. It also said the rules would ensure a fair return for federal taxpayers, tribes and states who earn royalties on the sale of gas.  Two industry groups, the Western Energy Alliance and the Independent Petroleum Association of America, immediately filed suit challenging the rule as an “11th hour shot by an administration that doesn’t fully understand how its rules impact our businesses.” The industry groups argued that such regulations were in the purview of the EPA, not BLM.

“We’ve been interested to see that the Congressional Review Act is being seriously discussed as an option for rolling back the barrage of regulations directed at all industries, not just oil and natural gas,” said Kathleen Sgamma, vice president of government and public affairs for the Western Energy Alliance. “We believe everything released since late May should be considered for roll-back.”

For regulations finalized before May—including some opposed by the energy industry—the Trump administration likely will face legal challenges that will slow or derail his deregulatory plans. Rules that are still being written are the easiest to block, by simply ordering the agency to stop working on them.

Here are some of the Obama administration rules that may be lined up on Trump’s chopping block:

Clean Power Plan (Aug. 3, 2015)

The centerpiece of the Obama administration’s action plan on climate change, the Clean Power Plan was put on hold by the Supreme Court earlier this year while the federal courts decide a legal challenge led by coal-dependent states. The Trump administration could choose to allow the legal challenge to play out. The case is certainly headed to the Supreme Court, and naming a justice to fill the high court’s empty seat is also on Trump’s 100-day agenda. But the administration also could file a motion to withdraw the rule.

If the court allowed it, the EPA would have to go through the public comment process to revise or eliminate the rule. Congress also could pass a law overturning the rule, but that likely would require 60 votes in the Senate, meaning Republicans would need the support of at least nine Democrats. It may be easier for Congress to cut funding for the EPA to implement the rule, since budget measures require only a simple majority.

Waters of the U.S. Rule (Aug. 28, 2015)

One of the fracking industry’s top priorities for the Trump chopping block is the so-called WOTUS rule, which was the EPA’s attempt to clear up confusion over federal wetlands protection. EPA administrator Gina McCarthy said the rule extended the reach of the Clean Water Act by about 3 percent, to encompass streams and wetlands.

(Opponents complained—falsely, according to EPA—that the agency’s new jurisdiction encompassed ditches and puddles.)  Energy producers strongly oppose the new permitting and regulatory requirements and North Dakota has led a legal challange to the rule.  A federal appeals court issued a stay on the regulations last October while that case proceeds. 

Methane Rule: New Facilities (May 12, 2016)

This regulation requires energy producers to systematically monitor gas leaks and make repairs at new oil and gas industry sites to prevent methane leaks. Producers maintain such rules are unnecessary; since methane is essentially natural gas, they say they already have an incentive to capture all they can for sale as fuel. But the EPA calculates that methane leaks from hundreds of thousands of sources, including pneumatic devices, storage tanks and production fields added up to 9.8 million metric tons in 2014, an 11 percent increase from four years earlier. Getting control of methane, a potent greenhouse gas that can be controlled with existing technology, was a key part of the Obama administration’s plan for meeting its pledge under the Paris climate accord.

Methane Rule: Existing Sites (Underway)

At the same time that EPA issued its methane rule for new facilities, it began the much larger job of developing regulations for existing sites—an estimated 700,000 facilities.  The EPA has been attempting to finish the first step of that process, collecting data from the oil and gas industry, before the new administration takes office. The agency released its final information collection request two days after the election.

“EPA’s rush to get the Information Collection Request done before January 20^th is a fool’s errand, as it is not well thought out and not likely to survive past that date,” said Kathleen Sgamma, vice president of government & public affairs for the Western Energy Alliance. “Besides being an onerous exercise in gathering irrelevant information, it is impractical in many situations.”  It will be easy for the Trump administration to stop this rulemaking in its tracks, although it could face legal action for failure to live up to its duty to control the pollutant under the Clean Air Act.

Blowout Preventer Rules (April 14, 2016)

In response to the BP Deepwater Horizon disaster in the Gulf of Mexico in 2010, the Interior Department finalized rules to improve safety in the offshore drilling industry. The rules include new requirements for blowout preventers, well design, well control casing, cementing, as well as real-time monitoring and subsea containment. But the Independent Petroleum Producers of America called it a “highly prescriptive rule” that could result in “reduced safety, less environmental protection, fewer American jobs, and decreased U.S. oil and natural gas production.”

Greater Sage Grouse Protections (Sept. 25, 2015)

The Obama administration tried to strike a middle ground in addressing one of the most contentious Western land use decisions it faced: protection of the Greater Sage Grouse. The U.S. Fish and Wildlife Services decided that endangered species protection for the gaudy rangeland bird was not warranted; instead, the bird and its sage-steppe ecosystem would be protected through what the agency’s director Daniel Ashe called an “epic-scale partnership” among the federal and state governments, industry groups and private landowners to balance drilling and conservation.  

The Bureau of Land Management and the U.S. Forest Service issued 98 land-use plans in 10 Western states. In practice, it has meant that oil and gas lease sales have slowed, which Montana Petroleum Association executive director Alan Olson earlier this year said amounted to “more excuses not to allow development on public lands.”

Hydraulic Fracturing on Federal Lands (March 20, 2015)

In June, a federal court in Wyoming struck down the Obama administration’s permitting regulations to improve safety of hydraulic fracturing on federal lands. The rules required disclosure of fracking chemicals, monitoring of cementing operations during well construction and other safeguards. The administration has appealed; the Trump administration could opt to drop that appeal.

Coal Ash Regulations (April 17, 2015)

For decades, federal authorities have grappled with the issue of how to address one of the largest industrial waste streams in the United States: coal ash. This residue of coal combustion is laced with mercury and heavy metals stored in landfills and surface impoundments, typically at power plant sites. The effort to write regulations did not begin in earnest until a catastrophic breach at a Tennessee Valley Authority (TVA) power plant in Kingston, Tenn. flooded more than 300 acres of land and released coal ash into the Emory and Clinch rivers in 2008.

Environmental groups challenged the rules in court, saying they did not go far enough to reduce catastrophic failure and protect groundwater. An industry group representing coal utilities also mounted a legal challenge last May, saying the EPA acted beyond its legal authority.

Crystalline Silica Worker Safety Rule (March 23, 2016)

Sand is a key ingredient in fracking; it is shot at high pressure into wells to prop open the fissures in shale rock to release oil and gas. More than 11,000 U.S. oil and gas workers are being exposed to unsafe levels of silica dust due to the use of fracking sand, according to the U.S. Occupational Safety and Health Administration. They are among 940,000 construction and industrial workers that OSHA sought to protect from lung cancer, chronic respiratory disease, and kidney disease with a rule requiring dust control, monitoring and medical exams for workers.

But oil services company Halliburton said workers already were protected by respirators, and it warned “the entire industry will face significant compliance problems and costs” in comments filed with OSHA. The Greater North Dakota Chamber of Commerce, which represents fracking companies, is among several industry groups that have sued to block the regulations.

Correction: A previous version of this story incorrectly stated that the Waters of the U.S. rule was being blocked in 13 states. It is under injunction nationwide.