  A number of naturally occurring, proton-rich nuclides (the p-nuclei) cannot
be made in the s- and r-process. It has been found that massive stars can
produce p-nuclei through photodisintegration of pre-existing intermediate and
heavy nuclei. This so-called gamma-process requires sufficiently high
temperatures and occurs in pre-explosive or explosive O/Ne burning, depending
on the mass of the star. Although the gamma-process has been successful in
producing a large range of p-nuclei, two mass regions remain problematic, A<110
and 150<A<165, where a number of p-nuclei are severely underproduced. The
origin of the problems is yet to be identified. A large number of unstable
nuclei with only theoretically predicted reaction rates are included in the
reaction network and thus the nuclear input may involve uncertainties.
Deficiencies in charged-particle optical potentials at gamma-process
temperatures have been found for nuclei at stability. On the other hand, the
gamma-process conditions (temperature profiles, entropy of the O shell, seed
composition) also sensitively depend on details of the stellar structure and
evolution, as well as on the initial metallicity. Nevertheless, especially the
deficient low-mass p-nuclei may call for an additional production process or
site, such as production in (subChandrasekhar) type Ia supernovae. Also the
(im)possibility of a synthesis in the rp- and nu-p-processes is discussed. Were
this the case, the production of p-nuclei would be realized as a superposition
of several different processes, not necessarily in the same site.
