Before registration of births, marriages and deaths was introduced in 1837, parish registers are the primary source for family historians. Even after 1837, using parish registers of baptisms, marriages and burials can be a useful alternative to the civil records.
Southampton Archives holds Church of England parish records for Southampton, including original registers of baptisms, marriages and burials. At their most numerous there were 33 parishes in Southampton. The dates earliest register dates from 1552 and some parishes have deposited registers as recent as the 21st century.
All of the registers before 1837 are transcribed and indexed. Volunteers are in the process of compiling a surname index for registers after 1837 which for most parishes has been completed up to the early 20th century. This means that you don't need to know which parish your ancestor lived in to find an entry.