In certain alloy systems a random solid solution known as the disordered state, transforms into a regular array called the ordered state, below a critical temperatureTc.Over the past decade or so, a considerable controversy has arisen over the transient path(s) by which a disordered alloy is replaced by an ordered structure at stoichiometric compositions, and also concerning the nature of the phases under equilibrium conditions when the alloy departs from the ideal composition. The present review is an attempt to unify the different approaches with reference to both theoretical grounds and experimental findings.