The first part of this chapter explores key theories of sexual offending. It covers single-factor and multifactor models and presents both traditional rationales—such as learning theory and psychodynamic approaches—and a dedicated criminological perspective. The second part details the state of research on the impact of offender treatment on criminal recidivism. What are the core intended treatment outcomes for persons convicted of sex offenses? And what treatment concepts are state of the art, including cognitive-behavioral therapy, the Reasoning and Rehabilitation program, the Relapse Prevention approach, and broader, basic treatment rationales such as the Risk-Need-Responsivity Model and the Good Lives Model? This study having been conducted in German prisons, a subchapter is dedicated to research on social therapeutic treatment in the German penal system against the backdrop of data on the effectiveness of sex offender treatment at an international level.