  Analyses of comprehensive sets of strong-interaction data consisting of level
shifts, widths and yields in strange atoms of K mesons and Sigma hyperons are
reviewed. The introduction of density dependent optical potentials, in both
cases, improves significantly the fit to the data. A strongly attractive
antikaon-nuclear potential of order 150-200 MeV in nuclear matter is suggested
by fits to the kaonic-atom data, with interesting possible repercussions on
kaon condensation and on the evolution of strangeness in high-density stars.
The case for relatively narrow, deeply bound antikaon atomic states is made,
essentially independent of the nuclear potential depth. Dynamical models for
calculating binding energies and widths of antikaon-nuclear states are
discussed and a lower bound of approximately 50 MeV on the width is
established. For Sigma atoms, the fitted potential becomes repulsive inside the
nucleus, in agreement with recently reported (pi-,K+) spectra from KEK,
implying that Sigma hyperons generally do not bind in nuclei. This repulsion
significantly affects calculated compositions and masses of neutron stars.
