<p>The number of long-term cancer survivors is steadily increasing due to advances in early detection and treatment, and with it the number of people requiring treatment for a host of cancer-related complications. Among the most common complications is chronic pain, defined as pain that persists for three or more months. Chronic cancer related pain is in turn associated with an increased risk of fatigue, depression, anxiety, being unable to work, and needing assistance with activities of daily living. Guidelines for the management of chronic cancer-related pain recommend supplementing pharmacological with behavioral interventions, based on studies evaluating cognitive-behavioural therapies (CBT). One of the third-wave interventions of CBT is Acceptance and Commitment Therapy (ACT) which is a form of CBT that emphasizes increases in psychological flexibility through processes of acceptance, mindfulness, commitment, and behavior change. Both face-to-face and internet-delivered ACT have been found to be effective for chronic, non-cancer pain, with significant improvements in functioning, with gains maintained 3 or more months after treatment. There is also preliminary evidence that pain-focused CBT can help improve overall functioning in cancer patients, and that psychological flexibility is correlated with pain, fatigue, emotional and overall functioning in cancer patients and survivors. It is widely recognized that chronic pain in cancer patients and survivors is under treated in primary and specialist care settings in Sweden and across the globe. This is partly due to the limited availability of therapists who can provide pain-focused CBT programs, and this treatment being offered in specialist pain rehabilitation clinics that do not routinely accept individuals with cancer-related pain.</p><p>There is a need for low-cost, yet effective treatments to support people with cancer closer to home, and with input from healthcare providers outside of specialist clinics. Digitally delivered interventions, either via smartphone, tablets, computers, or teleconferencing, often collectively referred to as mobile-health interventions (mHealth), are seen as a cost-effective method for increasing access to treatment for cancer patients and survivors.</p><p>Also, there is now widespread acknowledgement that further improvements in the efficacy of CBT, including for chronic pain, will require research designs that allow the investigator to identify processes that underpin change in symptoms and overall functioning, and at the individual patient level, permitting greater personalization of treatments.</p><p>The main objective of this study is to test the feasibility, acceptability, and preliminary efficacy of an internet-delivered, therapist-assisted, CBT program based on the psychological flexibility model of chronic pain, with adults suffering from chronic cancer-related pain and associated difficulties. Specifically, the investigators aim to: a) test whether the intervention is feasible and acceptable; and in a preliminary fashion, b) to test the efficacy of the intervention for the primary (pain interference) and secondary outcomes (pain intensity, fatigue, fear of cancer recurrence, depression and anxiety, functioning, well-being, and psychological flexibility) when compared to a no-treatment randomized baseline period; c) to test if any therapeutic gains are maintained at 3-month follow-up; d) to test if changes in outcomes from pre-treatment to the end of treatment and follow-up are mediated by changes in psychological flexibility; and e) to examine outcomes at both the individual and group-level. To achieve the project&#x27;s goals, a non-concurrent, randomized, multiple-baseline across participants, single case experimental design will be used.</p>