The concept of the core developmental task, which is fundamental to assessment and therapeutic communication, is of particular importance when we work with children who have experienced separation, loss and trauma. This is because development is malleable and in¯uenced by the relational and social context. In the course of our assessment, we may ask ourselves what may reasonably be expected from a child and young person at their particular age and stage, and what developmental and behavoural milestones would be considered to be appropriate. Where there is a lack of ®t between the age and stage of the child and their behaviour, we may go on to ask ourselves what factors in the child's experience and relationships have impeded or undermined the developmental task. In other words, we recognize that these differences or gaps are of a quantitative, not a qualitative, nature. In this respect, the evidence on the impact of separation loss and trauma from research and clinical practice is unequivocal in asserting the importance of good early attachments and the provision of a secure relational base to the development of the child.