This article addresses the acquisition of Czech idioms in a Czech monolingual boy from speech onset until age 7. This study is based on an analysis of authentic material: a corpus of transcriptions of audio recordings of the child in verbal interaction with adults, and diary records. The acquisition sequence and types of acquired idioms were analysed. Also the source of the idioms (family, media, school) was taken into consideration. One of the most important aspect analysed is the context when the child uses the idiom and the way he understands it which is sometimes different from adult usage.