In the very early stages of embryonic development, cells have the capability of dividing indefinately and then differentiating into any type of cell in the body. Recent studies have revealed that much of this remarkable developmental potential of embryonic stem cells is retained by small populations of cells within most tissues in the adult. Intercellular signals that control the proliferation, differentiation and survival of stem cells are being identified and include a diverse array of growth factors, cytokines and cell adhesion molecules. Intracellular mechanisms that regulate stem cell fate are also emerging and include established second messenger pathways, novel transcription factors and telomerase. The possibility that a decline in the numbers or plasticity of stem cell populations contributes to aging and age-related disease is suggested by recent findings. The remarkable plasticity of stem cells suggests that endogenous or transplanted stem cells can be 'tweaked' in ways that will allow them to replace lost or dysfunctional cell populations in diseases ranging from neurodegenerative and hematopoietic disorders to diabetes and cardiovascular disease.