The introduction of stable isotope analysis (carbon, nitrogen, hydrogen, sulfur) of bones and food residues allows anthropologists to define more precisely the actual consumption patterns of extinct populations. However this requires that the ranges of compositions of possible foods be known; that there be isotopic variability in these foods (although some information about trophic-levels can be obtained for consumers of isotopically monotonous foods); that isotopic offsets (fractionations) between diet and sample (e.g., collagen) are known; and that samples are well-preserved. The dietary proportions of N+1 foods can be determined if isotope ratios are measured for N elements and if no three foods are co-linear in delta-space. Studies of ancient human populations from North and Central America are used to show that: variation in diet within a single time plane for a given culture is very limited except possibly where status differences occur; variation in diet through space and time can be easily recognized and may in some cases be related to independently inferred historical or environmental factors.