For heavy-section steels and weld metals used in nuclear reactor vessels, a sound understanding of fracture toughness in the transition range is needed. Fractographic studies of various kinds have been helpful. The results given in this paper indicate that the high local stress necessary to initiate cleavage is provided by brittle separations of closely spaced particle arrays, termed “particle clumps,” and, occasionally, in low-carbon weld metal by fracture of individual silicate particles 2 to 3 μm in size. These features, however, are far too numerous to account for the “rare event” nature of cleavage initiation in small-specimen tests. The increase of fracture toughness with test temperature across the transition temperature range is accompanied by an increase in size of late-breaking connections. This may be largely because of an increase with temperature of resistance to smooth passage of cleavage through grain boundaries.