The most probable candidate site for the r -process has long been suggested to be core-collapse supernovae (spectral types Ib, Ic and II), which may provide the necessary physical conditions for the r -process. However, the very low abundance of r -process nuclei in the interstellar gas limits the amount each can have ejected. It requires either that only a small fraction of supernovae eject r -process nuclei to the interstellar medium, or that each supernova ejects only a very small amount of r -process material. The ejected material must be relatively neutron-rich, a condition which has been difficult to achieve in models, so that astrophysicists remain uneasy about their adequacy for successful r -process yields.