Abstract-Idioms are an important part of a language, as they are used to express ideas in a more concise and effective way. Idioms have a considerable role in a foreign language as in the mother tongue. Therefore, their learning and teaching in L2 are worthy of attention and research. This paper intends to define idioms, introduce their different taxonomies, discuss L2 idiom processing and comprehension, and use the information to endow English teachers with some pedagogical implications.Index Terms-idiom taxonomies, idiom comprehension, literal and figurative meaning, opaque and transparent idioms