Aging is the most complex phenotype for a multicellular organism. This process is now being under severe investigation. Here I will review the different processes known to affect longevity in the nematode Caenorhabditis elegans and their relationship with thermotolerance. All the longevity mutants that have been tested so far show an increase in stress resistance. In particular, long-lived mutants affected in the IGF/insulin pathway and those affected in the germ-line formation are both thermotolerant and long-lived. The mechanisms that activate the stress resistance are now been understood including the DAF-16 fork head transcription factor transport to the nucleus and the activation of genes involved in the defense to stress. The high correlation between stress resistance and longevity suggests that the same molecular activities that defend the cell from stress can defend the cell from the damage caused by aging.