The cytotoxic T-lymphocyte–associated antigen 4 (CTLA-4) immune checkpoint is a negative regulator of T cell immune function. Compared to the cluster of differentiation (CD)28 receptor, CTLA-4 molecules bind to CD80 and CD86 with a higher affinity; thus, CTLA-4 molecules act as potent, competitive inhibitors of CD28 in antigen-presenting cells. Immune checkpoint pathways include an assortment of inhibitory pathways within T cells that substantially minimize collateral tissue damage and maintain self-tolerance. It is now well documented that cancer interacts with specific immune checkpoint pathways as an effective strategy of immune elusion. The blockade of these immune checkpoint pathways with antibodies is one of the most promising strategies for activating therapeutic antitumor immunity. Ipilimumab, a CTLA-4 inhibitor, was the first immunotherapeutic agent to receive US Food and Drug Administration approval. Although it has not attained formal approval, tremelimumab, a novel CTLA-4 immune checkpoint inhibitor, has also achieved promising results. This book chapter summarizes the current knowledge regarding the mechanisms and efficacy of CTLA-4 inhibition. It also discusses the results of immunotherapy administration in many clinical trials for a plethora of malignancies, including melanoma; lung, hepatocellular, colorectal, ovarian, biliary tract, prostate, and esophagogastric cancers; sarcoma; lymphomas; mesothelioma; and urothelial, renal cell, and oral carcinomas. Finally, it discusses biomarkers for predicting the response to immunotherapies, providing a comprehensive overview of the more recent platforms developed to achieve this goal. There is emerging evidence that manipulating the CTLA-4 immune checkpoint is a promising strategy for immunotherapy for many different types of cancer.