Synchronization expressions (SEs) were originally developed as practical high-level constructs for specifying synchronization constraints between parallel processes. The family of synchronization languages was introduced to give a precise semantic description for synchronization expressions. In addition to its use for defining the meaning of SEs, the family of synchronization languages is interesting on its own from a formal languages point of view. We consider two variants of the definition of synchronization languages, and survey characterization results for the language families. Synchronization languages also provide us a systematic approach for the implementation and simplification of SEs.