The relationship between microstructure and cleavage resistance in quenched and tempered high strength bainitic and martensitic steels is investigated by means of Charpy-V three-point bending tests, uniaxial tensile test on unnotched specimens and EBSD. Steels under investigation are low/medium carbon (C=0.10%-0.40%) steels with yield strength in the range YS=500-1000 MPa. Results show that the tensile strength and the cleavage resistance of Q&T steels appear to be controlled by different structural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit. In particular, yield strength is controlled by the mean subgrain size, whereas the structural unit controlling the critical cleavage stress is the covariant (bainitic or martensitic) packet, whose size is slightly lower than the average unit crack path (UCP). The critical stage in the fracture process appears to be the propagation of a Griffith crack from one packet to another, and the resistance offered by high-angle boundaries is approximately the same as that of low-C steels with bainitic or polygonal ferrite microstructure.