"In the past, Congress has regularly acted to extend expired or expiring temporary tax provisions. Collectively, these temporary tax provisions are often referred to as 'tax extenders.' Fifty-two temporary tax provisions expired at the end of 2014. This report provides a broad overview of the tax extenders. Congress last acted on tax extenders towards the end of the 113th Congress. The Tax Increase Prevention Act of 2014 (P.L. 113-295), signed into law on December 19, 2014, made tax provisions that had expired at the end of 2013 available to taxpayers for the 2014 tax year. The law extended most (but not all) provisions that had expired at the end of 2013. Most of the provisions in P.L. 113-295 had been included in previous 'tax extender' packages. Tax extenders legislation has also been considered in the 114th Congress. The Senate Finance Committee has reported legislation, the Tax Relief Extension Act of 2015 (S. 1946), that would retroactively extend expired tax provisions, for two years, through 2016. All provisions in S. 1946 have been included in previous 'tax extender' packages. There are several reasons why Congress may choose to enact tax provisions on a temporary basis. Enacting provisions on a temporary basis provides legislators with an opportunity to evaluate the effectiveness of tax policies prior to expiration or extension. Temporary tax provisions may also be used to provide temporary economic stimulus or disaster relief. Congress may also choose to enact tax provisions on a temporary rather than permanent basis due to budgetary considerations, as the foregone revenue from a temporary provision will generally be less than if it was permanent."
CRS Report for Congress, R43898
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html