Background: NENs include a heterogeneous group of tumors with different behavior. Despite immunomodulators such as interferon are approved for the management of grade 1-2 NENs, the low mutation tumor load and PD-1/PDL-1 expression have limited the development of immune check-point inhibitors in this setting. The rational for immunotherapy in high grade NENs is stronger compared with low grade NENs based on the results observed in small cell lung cancer. However, the combination of an antiPDL-1 and antiCTLA-4 could increase the probability of success regardless of tumor growth rate, mutational tumor load or PDL-1 expression in low grade NENs. Methods: This prospective, multi-center, open label, phase II study (EudraCT:2016-002858-20) will evaluate the efficacy and safety of durvalumab plus tremelimumab in 126 pts within four different cohorts, including well-moderately differentiated lung NENs (Cohort 1), grade 1-2 gastrointestinal NENs (Cohort 2), grade 1-2 pancreatic NENs (Cohort 3) and grade 3 GEP NENs (Cohort 4). To optimize the efficacy of immunotherapy in low grade NENs, pts included in the trial must have progressed to all standard approved therapy in each setting, including somatostatin analogues, targeted agents and chemotherapy for lung and GEP grade 1-2 NENs up to 4 prior lines. For pts included in cohort 4, progression to standard platinum-based chemotherapy is mandatory. All pts will receive durvalumab 1500 mg every 28 days for 12 months, and tremelimumab 75 mg Q4W up to 4 doses/cycles. Retreatment is allowed after disease progression in the follow-up period. The primary endpoint of the study for cohorts 1-3 is disease control rate at 9 months including the percentage of pts achieving complete response, partial response, or stable disease according to RECIST v1.1; Median overall survival will be the primary endpoint for cohort 4. Primary endpoints will be assessed by investigators and confirmed by central radiological review. Secondary endpoints include median progression-free survival, safety and tolerability and a wide panel of biomarkers in blood samples and tumor tissue. Clinical trial information: 2016-002858-20.