As of 2014, there are only 50 states that make up the Union; there is no 51st state. Hawaii became the 50th state in 1959, the same year that Alaska became a state.
While it has long been rumored that Puerto Rico could become a U.S. state, and a 2012 ballot indicated that many residents favor statehood, there is nothing indicating that it will happen any time soon. The U.S. gained control of the island nation in 1898 as a result of the Spanish-American War and granted citizenship to Puerto Rico's residents in 1917. However, those residents remain ineligible to vote in presidential elections.