After defining interlamellar spacing the various optical and electron optical methods for measuring spacing are outlined. It is clear for both isothermal and forced velocity transformation conditions that pearlite can grow at a constant velocity with a range of true spacings. The minimum true spacing and mean true spacing are not related by a constant factor, but this may vary from system to system and with temperature in a given system. The relationship between interlamellar spacing and temperature for isothermal growth conditions and between translation velocity and spacing for forced-velocity growth conditions is reviewed for a range of steels and nonferrous alloys. It is seen that the velocity-spacing relationship for the two modes of transformation is the same. For isothermal transformation a linear relationship between reciprocal spacing and temperature is generally observed, but for steels containing alloy additions there is little evidence of the predicted inflexion corresponding to a temperature at which alloy partitioning at the transformation front ceases. The lack of precise interfacial energy data makes it difficult to determine reliably the relationship between measured and critical spacings, although it seems likely to be in accord with the maximum growth rate or maximum rate of entropy production optimization criteria.