Cyclin-dependent kinase 11 (CDK11) is a multifunctional serine/threonine protein kinase that plays a pivotal role in transcription and pre-mRNA splicing. It phosphorylates serine 2 of RNA polymerase II C-terminal domain, thereby promoting transcriptional elongation and 3′-end processing of replication-dependent histone genes, as well as contributing to proper chromosome segregation during mitosis. CDK11 is essential for global pre-mRNA splicing by phosphorylating Splicing Factor 3B Subunit 1, a core U2 small nuclear ribonucleoprotein component, thereby activating the spliceosome. Since splicing is closely linked to optimal transcription and cell proliferation, inhibition of CDK11 is hypothesized to indirectly disrupt general transcription and cell cycle progression. CDK11 drives cancer cell proliferation, promotes HIV-1 mRNA 3′-end processing to enhance viral replication, and contributes to tau phosphorylation in Alzheimer’s disease. Given its integral role in key cellular processes and its dysregulation in various diseases, CDK11 has emerged as a compelling therapeutic target. This review provides a comprehensive overview of the biological functions and regulatory mechanisms of CDK11, discusses its role in cancer, viral and neurodegenerative diseases, and highlights advances in the discovery and development of CDK11 inhibitors, including OTS964, which has expanded our understanding of the biological functions of CDK11 and its prospects as a cancer-specific vulnerability.