Investigations of the relationship between language and German colonialism are mainly based on historical sources. The article aims to develop a systematic foundation of source studies as a methodological background for these investigations. This is exemplified by sources reflecting the particular situation of the former German colonies in the South Pacific. Firstly, the article addresses terminological problems, in particular the relation between “documents”, “sources”, and “data”. Secondly, a detailed typology of historical sources is presented and related to object-, meta-, and extralinguistic aspects of language contact. Finally, the article informs about how and where to look for historical sources.