Adult hippocampal stem/progenitor cells (AHPs) continuously give rise to new neurons throughout life, which might be an important determinant of hippocampus-dependent function. Strikingly, the fate potential of AHPs is not restricted to the neuronal lineage because AHPs can be genetically induced to generate oligodendrocytes within their in vivo niche by AHP-specific ectopic expression of the basic-helix-loop-helix (bHLH) transcription factor achaete-scute complex-like 1 (ASCL1). Fate plasticity of AHPs is controlled by cell-autonomous and also niche-dependent mechanisms. Here, we discuss the biological importance and potential therapeutic applications of retained fate plasticity of AHPs in the adult mammalian brain in addition to the future scientific inquiries indicated by this finding.