In the first part of the present review paper, experimental results which
lead to the discovery of two-proton radioactivity are reviewed. Beyond
two-proton emission from nuclear ground states, we also discuss experimental
studies of two-proton emission from excited states populated either by nuclear
beta decay or by inelastic reactions. In the second part, we review the modern
theory of two-proton radioactivity. An outlook to future experimental studies
and theoretical developments will conclude this review.Comment: review on two-proton radioactivity, 50 pages, 18 figure