Precipitation from solid solutions and supersaturated solutions is a very important process as it plays an important role in the structure and properties of metals/alloys. This process is used widely in the metallurgical engineering to obtain desired mechanical properties. The phenomena under discussion has great practical significance, for they make possible the precipitation or age hardening of metals, an extremely important way of hardening of metals, especially in commercial strengthening of non-ferrous alloys, e.g. aluminum and magnesium alloys. This review discusses precipitation and its effects on hardening in solid solutions and saturated solid solutions.