This chapter introduces the term third-wave psychotherapies and provides a concise overview of the historical evolution of the three major waves of psychotherapeutic practice: psychoanalysis, behavioural, and cognitive-behavioural therapies, and the more contemporary process-oriented third-wave approaches. It examines the key theoretical and clinical developments that have shaped each wave, drawing attention to a fundamental shift in the understanding and treatment of psychological well-being. Whereas earlier models often focused on the content of thoughts, symptoms, or past experiences, third-wave approaches emphasise the processes by which individuals relate to their internal experiences. Central to this paradigm shift is the recognition that psychological distress is not solely caused by the presence of painful thoughts and emotions, but is often perpetuated by individuals’ rigid attempts to control, suppress, or avoid these experiences. By fostering emotional regulation, acceptance, mindfulness, and psychological flexibility, third-wave therapies offer a transformative perspective on mental health, moving beyond symptom reduction towards promoting meaningful engagement with life.