Long-term conditions like cancer can significantly impact one’s wellbeing and quality of life. Following a cancer diagnosis, individuals often experience a myriad of physical and psychological symptoms, including fear, guilt, shame, anger, anxiety, pain, fatigue, insomnia, depression, loss of meaning in life, and fear of cancer recurrence. Acceptance and Commitment Therapy (ACT) is an evidence-based transdiagnostic psychotherapy approach suitable for various conditions, including cancer. Grounded in contextual behavioral science, ACT enhances psychological flexibility by fostering processes such as valued living, acceptance, and mindfulness. By cultivating psychological flexibility, ACT empowers individuals to engage in meaningful activities despite the challenges of living with cancer, such as stressful thoughts and emotions, physical symptoms, and disruptions to daily life caused by treatment and recovery. This chapter outlines the theoretical basis of ACT and describes its use in patients with cancer.