Studies in Caenorhabditis elegans demonstrate that disruption of the daf-2 signaling pathways extends lifespan. Similarities among the daf-2 pathway, insulin-like signaling in flies and yeast, and the mammalian insulin-like growth factor 1 (IGF-1) signaling cascade raise the possibility that modifications to IGF-1 signaling could also extend lifespan in mammals. In fact, growth hormone (GH)/IGF-1-deficient dwarf mice do live significantly longer than their wild-type counterparts. However, multiple endocrine deficiencies and developmental anomalies inherent in these models confound this interpretation. Here, we critique the current mammalian models of GH/IGF-1 deficiency and discuss the actions of GH/IGF-1 on biological aging and lifespan.