This qualitative case study reports and interprets school principals' perceptions of their role in implementing and sustaining Second Step, a social skills development programme. The study compares schools that implemented the programme as a formalised strategy for all teachers and those that did not. The results suggest that the key features of successful implementation are: (1) a strong, continued and systematic focus by principals on leadership, combined with specific management strategies, with an emphasis on both individual and organisational factors, in order to provide direction, secure the alignment of teachers and provide sufficient resources; and (2) the anchoring of the programme at both top and bottom levels of the school organisation. (Contains 1 table.)