After reviewing the behavior of a number of systems exhibiting discontinuous precipitation, the theoretical treatment of the process is discussed, with emphasis on the problem of the interlamellar (or inter-rod) spacing. The case of the minimum, or critical, spacing is discussed for lamellar and rod products, as is a possible criterion for the maximum accessible spacing. The problem of the optimal spacing is then addressed, mainly from an experimental point-of-view. Experimental data for Al-Zn and Mg-Al alloys are analyzed and related to the discussion.