New Jersey State House, Trenton

The New Jersey Senate was established as the upper house of the New Jersey Legislature by the Constitution of 1844, replacing the Legislative Council. There are 40 legislative districts, representing districts with an average population of 232,225 (2020 figure). Each district has one senator and two members of the New Jersey General Assembly, the lower house of the legislature. Prior to the election in which they are chosen, senators must be a minimum of 30 years old and a resident of the state for four years to be eligible to serve in office.
From 1844 until 1965 (when the Reynolds v. Sims US Supreme Court decision mandated all state legislators be elected from districts of roughly equal population), each county was an electoral district electing one senator. Under the 1844 Constitution, the term of office was three years, which was changed to four years with the 1947 Constitution. Since 1968 the Senate has consisted of 40 senators, who are elected in a "2-4-4" cycle. Senators serve a two-year term at the beginning of each decade, with the rest of the decade divided into two four-year terms. The "2-4-4" cycle was put into place so that Senate elections can reflect the changes made to the district boundaries on the basis of the decennial United States Census. If the cycle were not put into place, then the boundaries would sometimes be four years out of date before being used for Senate elections. Rather, with the varied term, the boundaries are only two years out of date. Thus elections for Senate seats take place in years ending with a "1", "3", or "7" (i.e. next elections in 2023, 2027, and 2031).

Interim appointments are made to fill vacant legislative seats by the county committee or committees of the party of the vacating person (since a constitutional amendment passed on November 8, 1988). The office is on the ballot for the next general election, even if the other Senate seats are not up for election in that year (such as in years ending with a "5" or "9", such as 2009 or 2015). The sole exception to this is if the vacancy occurred within 51 days of the election, in which case the appointment stands until the following general election.