With the advent of crystal settling in a solidifying igneous intrusion, the roof contact of the intrusion may be continuously subjected to temperatures approaching those of the magma itself. This causes steeper temperature gradients in the roof rocks and, consequently, more rapid heat loss and solidification than would occur if the crystals were frozen to the roof. By contrast, the accumulation of crystals in the lower parts of the intrusion acts to lower the floor contact temperature and so reduces temperature gradients in the floor rocks; but since the proportion of liquid near the floor is reduced, the overall effect also is to speed solidification, specifically in this case, from the floor upwards. The present paper reviews in a qualitative way the relation between crystallization mechanisms and temperature in a magma body when crystal fractionation is a major process. It then examines quantitatively, by means of one-dimensional solutions of the heat flow equation, the influence on crystal accumulation rates and roof rock temperatures of (a) variations in magma temperature, (b) pre-intrusion temperature gradients in the roof rocks, (c) shallow emplacement, (d) formation of an upper border zone or floating of crystals, and (e) latent heat effects associated with contact metamorphism. Finally, it considers the effects of heat loss to the floor rocks on (i) crystallization in the magma above the accumulating pile of crystals, (ii) solidification of trapped pore magma, and (iii) contact metamorphism of the floor rocks. Some of the more important equations presented also apply to sills, sheets, and dikes showing no crystal fractionation.