Purpose <p>Develop an instrument to assess unmet needs in cancer patients using immuno-, biological and precision (IBP) therapies.</p> Methods <p>Development followed COSMIN guidance. Instruments to assess unmet needs of advanced cancer patients were identified, and quality and content were evaluated in a systematic review (Phase 1). Semi-structured interviews with patients utilising IBP therapies (<i>n</i> = 31) and healthcare professionals (<i>n</i> = 22) explored supportive care needs (Phase 2). Phase 3 selected a base instrument to adapt, generated new items and iteratively refined these through six meetings involving professionals (<i>n</i> = 8) and public and patient involvement representatives (<i>n</i> = 9) and patient cognitive interviews (<i>n</i> = 16). Phase 4 piloted the new instrument (<i>n</i> = 50 patients).</p> Results <p>Twenty-four instruments were identified; none was developed for patients utilising IBP therapies (Phase 1). Ten domains of unmet needs were identified from the interview data (Phase 2). SCNS-SF34 was selected as the base instrument. Informed by interview data, an “add-on module” (SCNS-TARGET) was developed for patients utilising IBP therapies comprising 25 questions (psychological domain, 7 items; information, 6; healthcare, 5; economic, 3; role, 2; physical, 1; social, 1; Phase 3). Levels of missingness were low; reliability varied across questions, and, on average, patients reported 7.40 (standard deviation = 8.43) unmet needs on SCNS-TARGET (Phase 4).</p> Conclusions <p>SCNS-TARGET is designed for use alongside SCNS-SF34 to assess unmet needs in those using IBP therapies. Content and face validity have been established.</p> Implications for Cancer Survivors <p>SCNS-TARGET can help researchers and healthcare professionals determine unmet needs and inform requirements for new services and interventions, among patients using IBP therapies.</p>