Fate mapping was once the province of classical experimental embryologists. Now a battery of new and sophisticated methods can be used to trace where cells go and what they do in embryos. Here we use examples from gastrulating fish and amphibian embryos and from the chick limb bud and central nervous system to show how this information has contributed to our understanding of developmental processes. This knowledge will become increasingly important in interpreting the complex patterns of gene expression that are being discovered during development, as well as in understanding the effects of genetic manipulations and in directing experimental interventions.