Non-completion of treatment is a common occurrence in correctional contexts. Non-completion compromises service cost-efficiency, impacts adversely on staff and service user morale and may limit the effectiveness of therapy. Attention needs to be paid to enhancing offenders' readiness for treatment, and developing and maintaining their engagement. In this paper, we state that research and practice in offender treatment readiness and engagement need to be driven in four major ways: (1) the construction of models of engagement that are theoretically based and empirically evidenced that can underpin assessments and treatments; (2) the development of psychometrically robust assessments of treatment readiness, motivation and engagement that can be used to select offenders for treatment or measure change over time; (3) the design, implementation and evaluation of pre-treatment preparation procedures that can promote treatment engagement and completion; and (4) the development of strategies that address barriers to engagement as an integral part of treatment. This special issue contains contributions from leading researchers on the assessment of engagement, effects of engagement and non-engagement on outcomes, offenders' reasons for non-completion of treatment programmes, and the development of a measure of offenders' current concerns and life aspirations. These studies will improve research and practice in offender treatment readiness and engagement, and this could lead to better outcomes in terms of reduced re-offending.