This expert lecture will explore the importance of involving end users (families), in co-designing interventions intended to be used by them, if these interventions are going to be effective. The experiences of developing a safety netting intervention with, and for, families with acutely sick children will be used as an exemplar. This exemplar will illustrate the rationale for co-design, the evolving definition of, and need for, safety netting interventions and the processes involved. The complex interventions framework was used to structure the approach taken and its application will be delineated in each of the steps depicting the co-designed research programme.