This purpose of this thesis is to explore the relationship between idiomatic knowledge and second language proficiency. As the amount of research directly related to this topic is sparse, an in-depth discussion of relevant research and definitions comprises the first part of this paper. Two studies are then presented here that test the relationship between idiomatic knowledge and second language proficiency. A new definition for idioms proposes that all non-compositional phrases, popularized by usage, that is stored, retrieved, and employed as a single lexical unit. The results from more than 400 participants across two studies indicate that the two constructs are strongly correlated, but that the observable relationship between them is only modest. Additional results from the two studies also indicate that idiomatic knowledge is more strongly correlated with speaking skill than with writing or reading skills. The impacts of this study on existing research are discussed and directions for new research is suggested.