The use of immune checkpoint inhibitors has emerged as an effective treatment option for patients with several tumor types. By increasing the activity of the immune system, they can induce inflammatory side effects, which are often termed immune-related adverse events. These are pathophysiologically unique toxicities, compared with those from other anticancer therapies. In addition, the spectrum of the target organs is very broad. Immune-inflammatory adverse events can be life threatening. Prompt diagnosis and pharmacological intervention are instrumental to avoid progression to severe manifestations. Consequently, clinicians require new skills to successfully diagnose and manage these events. These SEOM guidelines have been developed with the consensus of ten medical oncologists. Relevant studies published in peer-review journals were used for the guideline elaboration. The Infectious Diseases Society of America grading system was used to assign levels of evidence and grades of recommendation.