**Abstract**

Immune checkpoint inhibitors (ICIs) are a type of cancer immunotherapy that has provided a tremendous breakthrough in the field of oncology. Currently approved checkpoint inhibitors target the cytotoxic T-lymphocyte-associated protein 4 (CTLA4), programmed death receptor-1 (PD-1), and programmed death-ligand 1(PD-L1). One of the most known complications of these advances is the emergence of a new spectrum of immune-related adverse events (irAEs). In this chapter, we will focus on selected rare or very rare irAEs, shedding the light on the other side of the coin of personalized cancer immunotherapy. We will also discuss general management approach of irAEs with an in-depth look on each one of these rare irAEs. The chapter will also cover principles of immunotherapy rechallenge post-occurrence of irAEs, and the impact of irAEs incidence on the efficacy of ICI. We will discuss some of the rare or very rare irAEs including cutaneous irAEs, immune-mediated Hypophysitis, hematological irAEs, ophthalmic irAEs, checkpoint inhibitor pneumonitis (CIP), neurologic irAEs, infectious irAEs, and cardiac irAEs. This chapter tried to highlight the significance of identifying emerging rare and very rare irAEs while considering initial assessments and management approaches identified in various clinical practice guideline and primary literature data.

**Keywords:** immune checkpoint inhibitors, adverse effects, pharmacovigilance, rare, irAEs, cancer immunotherapy
