  One of characteristic phenomena for nuclei beyond the proton dripline is the
simultaneous emission of two protons (2\emph{p}). The current status of our
knowledge of this most recently observed and the least known decay mode is
presented. First, different approaches to theoretical description of this
process, ranging from effective approximations to advanced three-body models
are overviewed. Then, after a brief survey of main experimental methods to
produce 2\emph{p}-emitting nuclei and techniques to study their decays,
experimental findings in this research field are presented and discussed. This
review covers decays of short-lived resonances and excited states of unbound
nuclei as well as longer-lived, ground-state radioactive decays. In addition,
more exotic decays like three- and four-proton emission are addressed. Finally,
related few-body topics, like two-neutron and four-neutron radioactivity, and
the problem of the tetraneutron are shortly discussed.
