Musashi1 (Msi1) is a highly conserved RNA binding protein that was initially identified in Drosophila by its ability to regulate sensory organ development and asymmetric cell division. Studies in mammalian cells reveal multiple functions for Musashi1 in normal and abnormal processes by mediating different post-transcriptional processes. According to our recent studies, Musashi1 very likely controls hundreds of targets, forming networks that regulate apoptosis, differentiation, proliferation and cell cycle. Musashi1 is a characteristic stem cell marker that regulates the balance between self-renewal and differentiation. Over-expression of Musashi1 has been associated with numerous tumor types and its function is required for tumor growth in breast, colon, medulloblastoma and glioblastoma. Musashi1 has also been implicated in neurogenesis and neurodegenerative diseases, and is emerging as a potential therapeutic target in both regenerative medicine and cancer.