Since its discovery nearly a decade ago, apoptosis-inducing factor (AIF) has had anything but a staid and uneventful existence. AIF was originally described as a mitochondrial intermembrane protein that, after apoptosis induction, can translocate to the nucleus and trigger chromatin condensation and DNA fragmentation. Over the years, an AIF-mediated caspase-independent cell death pathway has been defined. Rather than functioning as a general component of the cell death machinery, AIF is required for specific cell death pathways, including lethal responses to excitotoxins such as N-methyl-D-aspartate and glutamate, the DNA-alkylating agent N-methyl-N'-nitro-N-nitroso-guanidine, hypoxia-ischemia, or growth factor deprivation. Also, important roles of AIF in mitochondrial metabolism and redox control, and more recently in obesity and diabetes, have been discovered. Much of our knowledge has come from studies of AIF orthologs in model organisms, Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, and mice, which have also highlighted the importance of AIF in animal physiology and human pathology. Here, we discuss the manifold nature of AIF in cell life and death, with particular emphasis of its roles in vivo.