The bill makes cuts and adds exemptions to individual, corporate and international taxes, but only a few are permanent. Here's what you need to know about when the changes go into effect — and for how long.
Individual tax code changes
Four changes to the individual tax code go into effect on Monday but expire at the end of 2025: individual tax cuts, the expanded child tax credit, a doubled exemption for estate taxes and an increased exemption for the alternative minimum tax.
A retroactive increased deduction for medical expenses applies for a two-year period, from the beginning of 2017 to the end of 2018.
Beginning permanently in 2019 are the end to deductions for alimony payments for new divorces and the repeal of the individual mandate for Obamacare.
Corporate tax code changes
The business world will see the corporate tax rate drop permanently to 21 percent, from 35 percent, beginning Monday.
Additionally, starting in 2018 interest deductions will be limited to 30 percent of earnings before interest, taxes, depreciation and amortization. And starting in 2022, the 30 percent in interest deductions will no longer include depreciation or amortization expenses. Under the current rules, companies can deduct 100 percent of interest payments from taxable earnings.
Retroactive from Sept. 27 and extending to the end of 2022 is a period of full and immediate expensing — an allowance for businesses to deduct the cost from assets acquired instantly. That will be phased out, at a 20 percent reduction per year, until it ends in 2026.
Lastly, starting in 2022, companies will no longer be able to write off their full research and development costs immediately. Instead, they'll have to spread those expenses out over five years, a change that is estimated to raise nearly $120 billion in tax revenue over the next decade.
International tax code changes
Companies will have to begin repatriating their foreign earnings starting in 2018, but they have the option of paying the bill over eight years. They'll get a special one-time tax rate of 8 percent of illiquid assets and 15.5 percent for cash.
In addition, the tax law creates new rules intended to prevent companies or foreign subsidiaries located in the United States from shipping their profits offshore. The base erosion and anti-abuse tax will go into effect next year at a 5 percent rate, then jump to 10 percent in 2019. By 2026, the rate will rise once more to 12.5 percent.
— CNBC's Ylan Mui contributed to this report.