The analyses of the strangeness exchange (K^-,\pi ) and the associated production (\pi ^-,K⁺) reactions are presented. They indicate --- together with the observed properties of  \Sigma  atoms --- that the  \Sigma  single particle potential V_\Sigma is repulsive inside nuclei and has a shallow attractive pocket at the nuclear surface. This conclusion is consistent with the Nijmegen model F of the hyperon--nucleon interaction. It is demonstrated how the strong-interaction shifts and widths measured in  \Sigma  atoms may be used to obtain information on the nucleon density distributions.

PACS numbers: 21.10.Ft, 21.10.Gv, 13.75.Ev, 36.10.Gv