Background: Immune checkpoint inhibitors (ICIs) have improved cancer outcomes. However, immune-related adverse effects are common. The aim was to investigate the incidence of diarrhea and colitis of ICIs alone and in combination with chemotherapy or tyrosine kinase inhibitors (TKIs), histopathological findings, and management. Methods: Two separate studies, including meta-analyses, were performed. Key inclusion criteria were for Study I) phase I-IV trials, and data on diarrhea and/or colitis; for Study II) studies describing histopathologic and endoscopic findings and/or biologic treatment for ICI-induced colitis. Results: The incidence of anti-PD-1/PD-L1 antibody-induced diarrhea and colitis was 10% and 2%, respectively, with no clinically relevant differences between the compounds. The CTLA-4 inhibitor, ipilimumab, induced diarrhea and colitis in 33% and 7% of patients, respectively, whereas the incidence of diarrhea and colitis following ipilimumab combined with nivolumab was 21%-37% and 4%-8%, depending on regimen. The incidence of all-grade diarrhea following ICIs combined with chemotherapy or TKIs was high (17%-56%), whereas only 0.5% of patients developed severe (≥grade 3) colitis. The main patterns of histopathologic presentation after PD-1/CTLA-4 inhibitor mono- or combination therapy were acute and chronic active colitis and microscopic colitis-like. Infliximab and vedolizumab were equally effective against ICI-induced colitis. Conclusion: Expanding treatment options include combinations of ICIs and chemotherapy/TKI with a high incidence of diarrhea and a low incidence of colitis; thus, a potential risk of overtreatment with corticosteroids exists. We suggest a more tailored approach, particularly for the management of low-grade diarrhea. Prospective clinical trials are needed to refine management.