A review is given of the theoretical and experimental data on the isostructural first-order semiconductor-metal phase transition in samarium monosulfides and Sm$_{1-x}$Ln$_x^{3+}$S solid solutions (Ln is a rare-earth metal). Phase transitions occurring under hydrostatic pressures and as a result of change in temperature are considered. Electrical, magnetic, galvanomagnetic, thermoelectric, thermal, optical and mechanical properties are discussed, as well as the energy band structure of the semiconductor and metal modifications of the investigated materials. Special attention is given to the experiments confirming the fractional valence of the samarium ion in the metal phase of these materials (the experimental evidence for this valence is provided by the M\"ossbauer effect, magnetic susceptibility, shifts of x-ray $K$ lines, x-ray emission spectra, and specific heat).