It is now widely recognized that retrotransposons such as Long Interspersed Element-1 (LINE1 or L1s) have played an instrumental role in shaping the structure and function of our genomes. L1s contribute to ~17% of total human genome, and there is increasing evidence of its active role in neuronal development. L1s retro transposition was proposed as a potential mechanism of generating neuronal genome diversity, and multiple groups have attempted to characterize L1s retrotransposition events and their impacts on brain function and physiology. L1s are also linked to cognitive and psychiatric diseases such as Rett syndrome and schizophrenia. Here, I discuss the recent progress in characterizing the L1s retrotransposition in neuronal development, and how the knowledge has contributed to our understanding of brain function and diseases.