Foster carers require high quality evidence-based psychoeducational programs to support them in the care of children with complex trauma-related difficulties. However, there is a lack of systematic development of such programs which may explain mixed results. This paper presents a detailed account of the development of a complex intervention. This program was developed to address a practice gap of evidenced-based foster care programs in the Irish context. It aims to improve foster carers' capacity to provide children with trauma-informed care and in turn improve emotional and behavioural difficulties. The framework of the Medical Research Council (MRC) for the development and evaluation of complex interventions was used to develop Fostering Connections: The Trauma-informed Foster Care Programme. A prior narrative review of the evidence base of similar programs was combined with a prior qualitative study. A Stakeholder Group provided expert feedback during the development process. The development of a promising psychoeducational programs for foster carers using the MRC framework is described