Partly due to a lack of evidence-based methods to support people with intellectual disability (ID) and challenging behavior, their needs are often poorly met. One way to generate rapid evidence is to systematically describe and monitor interventions that are considered to be “good practice”—to develop evidence based on practical knowledge. This study describes the Dutch practice-based intervention Triple-C (Client, Coach, Competence). The intervention was developed in practice to support people with severe ID to borderline functioning and challenging behavior. The practice-based nature of Triple-C means that many of the professionals’ actions or activities are often underpinned by their implicit knowledge about the intervention they are delivering. Consequently, as the emphasis is on practice, the professionals can find it difficult to articulate how the intervention is operationalized and positive change achieved. This study aimed to assess the practical knowledge of Triple-C professionals and to develop an understanding of the mechanisms of change for Triple-C to improve understanding and to inform future research about the intervention. Through an iterative process, a logic model was developed to describe the intervention and its underlying assumptions. The development of the logic model was shaped using interviews with the founders, focus groups with support staff, psychologists, managers and members of the board of a service provider, and the analysis of published accounts of the Triple-C intervention. Data gathered from these sources were analyzed using content analysis. The logic model of the Triple-C intervention provides insight into the key elements of the approach, such as the need for unconditional supportive relationship and carrying out meaningful activities. Moreover, the potential relationship with existing evidence-based interventions such as Positive Behavioral Support and Active Support are described. Dening the underlying logic of a practice-based intervention like Triple-C is an important first step toward producing an evidence base for interventions developed from clinical practice.