  The Milky Way has been estabished to emit gamma rays. These gamma rays are
presumably dominated by decays of neutral pions, although inverse Compton
scatterings and bremsstrahlung also contribute. It is plausible that other
galaxies can be diffuse sources of gamma rays in a similar manner. Starburst
galaxies are particularly interesting to study as they are expected to have
much higher cosmic-ray fluxes and interstellar matter densities. The neutral
pions are created in cosmic-ray interactions with interstellar matter.
Presented here is an overview of the recent work by Karlsson and co-workers on
proton-proton interactions and the resulting secondary particle inclusive cross
sections and angular distributions. This model can be used to calculated the
$\pi^{0}$ component of the gamma-ray yield and spectrum from a starburst
galaxy. The yield is expected to increase significantly (30% to 50%) and the
spectrum to be harder than the incident proton spectrum.
