A review is presented of current theories of nucleosynthesis. The predicted contributions from (1) cosmological nucleosynthesis, (2) super-massive stars, (3) non-violent (quasi-static) stellar evolution, (4) supernova explosions, (5) cosmic ray interactions with the interstellar medium and (6) nova explosions to the observed solar system abundances are summarized. Recent studies of ‘explosive nucleosynthesis’ in supernovae and of the production of lithium, beryllium and boron by the interaction of cosmic rays with interstellar gas are emphasized. Observations of stellar spectra which either impose limitations upon or provide confirmation of various aspects of these theories are noted, as are several critical nuclear experiments. The general picture which emerges is incouraging in that most of the major abundance features appear to be at least qualitatively understood, but significant further research is required.