The Christian Social Union in Bavaria (CSU – Christlich-Soziale Union in Bayern e.V.) is a conservative political party in Germany. It operates exclusively in the state of Bavaria, while its sister party CDU runs in the rest of the country. Only in 1957 in the Saarland a CSU was running against a CDU, but this part of the CSU was merging with the CDU.
The CSU has led the Bavarian state government practically since its existence, and without the need for a coalition government for most of the time, which is, to this extent, unique in post-war Germany. On the federal level, it forms a common faction in the Bundestag (Federal Assembly) with the CDU. Edmund Stoiber took over the CSU chairmanship early in 1999.