The aim of this research is to look at effectiveness and its relevance and definition within the context of youth justice. Although ‘effective’ is frequently used to describe the success of youth justice policies, interventions and systems, its precise definition remains elusive. In order to explore the concept of effectiveness, and its use in the area of youth justice, an extensive review of the literature was undertaken, establishing a theoretical framework which provided the basis for a thematic analysis of Irish policy documents. This approach offered a detailed exploration and comprehensive insights into where the understanding of effectiveness within these documents sits in relation to the literature in the area. This analysis revealed that ‘effectiveness’ is a highly fluid and adaptable term, which is capable of meaning many things to many different stakeholders. It often serves as a catch-all phrase that is shaped by political, social, and ideological contexts. As a result, the concept is difficult to pin down and is open to interpretation depending on the perspective of the evaluator. While its position within youth justice remains the same, its meaning is everchanging and is not captured easily. There is a clear need for a more consistent and streamlined definition of effectiveness, with further research required in the area.