Stacking faults and dislocation dissociations on {011} and {121} planes in body-centred cubic lattices are examined. The effect of stress on the threefold dissociation of screw dislocations on {121} planes is studied in detail. A change in configuration occurs on reversal of stress, as in tension and compression experiments; this is used to explain slip asymmetry. Mechanisms for the initiation of glide on {011} and {121} planes and for cross-slip between the two types of planes are suggested. The model can explain the observed dependence of slip planes on orientation, temperature and sense of slip. The behaviour depends critically on the energies of the various types of stacking faults. Conditions for twinning to occur, rather than glide, are also determined.