Geminin was initially characterized as a bifunctional protein with roles in regulating the fidelity of DNA replication and in controlling cell fate during embryonic nervous system formation. More recently, Geminin's roles have expanded, encompassing regulation of cell proliferation and differentiation during retinogenesis, control of Hox transcription factor function during vertebrate axial patterning, and regulation of the timing of neuronal differentiation. Geminin interacts with homeodomain-containing transcription factors and with protein complexes that regulate chromatin structure, including Polycomb complexes and the catalytic subunits of the SWI/SNF chromatin remodeling complex, Brg1 and Brahma. Activities for Geminin in coordinating cellular events at the transition from proliferation to differentiation have recently emerged in multiple developmental contexts. This review will summarize Geminin's increasingly diverse roles as a developmental regulatory molecule.