The Northern Crusades or Baltic Crusades were Christianization campaigns undertaken by Catholic Christian military orders and kingdoms, primarily against the pagan Baltic, Finnic and West Slavic peoples around the southern and eastern shores of the Baltic Sea, and also against Orthodox Christian East Slavs.

Christianization is a term for the specific type of change that occurs when someone or something has been or is being converted to Christianity. Christianization has, for the most part, spread through missions by individual conversions but has also, in some instances, been the result of coercion from governments or military leaders. Christianization is also the term used to designate the conversion of previously non-Christian practices, spaces and places to Christian uses and names. In a third manner, the term has been used to describe the changes that naturally emerge in a nation when sufficient numbers of individuals convert, or when secular leaders require those changes. Christianization of a nation is an ongoing process.