Immune checkpoint inhibitor (ICI) treatments activate T cells against tumors. Activated T cells attack not only the tumor but also healthy cells, causing an autoimmune reaction in various tissues. These immune-related adverse effects (IRAEs) cause 18F-FDG uptake in various tissues due to inflammation. It is important to recognize and report these findings on 18F-FDG PET/CT studies. 18F-FDG PET helps to determine the presence, location, and severity of IRAEs. In severe cases, ICI treatments are interrupted or suspended and antiinflammatory treatments are started. 18F-FDG uptake due to IRAEs may mimic metastases or disease progression. Their presence may also help in predicting response to treatment and have prognostic implications. In this review article, we provide basic information about ICI treatments, IRAEs, and 18F-FDG PET/CT findings.