The US Research & Experimentation Tax Credit, R&D Tax Credit, was a temporary measure introduced in the 1980's. The US Congress has extended the measure multiple times but in December 2015 the President made it a permanent item of the US tax code with the passing of The Protecting Americans from Tax Hikes Act (PATH) of 2015 (the "Act").[1]
 
The measure provides certainty to claimants as they can be sure their R&D expenditures will be considered for eligibility in the future.
 
In addition to making the R&D Tax Credit permanent, eligibility has been extended.
 
The following amendments to the Internal Revenue Code (IRC) "apply to taxable years beginning after December 31, 2015".
 

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Making the R&D Tax Credit Permanent

The act strikes subsection 41(h) of the IRC "Credit for increasing research activities". The subsection contained a termination provision. 
 

R&D Tax Credit against AMT

 In general, a taxpayer is liable for the greater of their regular tax rate or the federal alternative minimum tax (AMT). With the passing of the legislation, eligible small businesses can apply the R&D tax credit against AMT, thereby lowering their overall tax payable.
 
Subsection 121(b) "Credit Allowed against Alternative Minimum Tax in Case of Eligible Small Business" amends 38(c)(4)(B) of the IRC to implement the credit against AMT. Per 38(c)(5)(C) "eligible small business" means a non-public corporation, a partnership or sole proprietorship, with gross annual receipts not exceeding $50,000,000 in the previous 3 taxable years.
 

R&D Tax Credit against Payroll Taxes

For certain small business, the credit can now be applied against any payroll tax owing. Subsection 121(c) "Treatment of Research Credit for Certain Startup Companies" amends section 41 of the IRC to add subsection "(h) Treatment of Credit for Qualified Small Businesses". A "qualified small business" means a corporation or partnership with gross receipts in the taxable year less than $5,000,000 (but excludes a tax-exempt corporations as defined in section 501 of the IRC). This credit against payroll tax is capped at $250,000 per taxable year for a maximum of 5 years.

Section by Section Summary of The Protecting Americans from Tax Hikes Act (PATH) of 2015[2]
  
1: http://docs.house.gov/billsthisweek/20151214/121515.250_xml.pdf
2: http://waysandmeans.house.gov/wp-content/uploads/2015/12/SECTION-BY-SECTION-SUMMARY-OF-THE-PROPOSED-PATH-ACT.pdf