Janus kinase (JAK) inhibitors are small synthetic molecules which can easily enter the intracellular space and inhibit janus kinases, which renders the janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway to be nonfunctional. This pathway is a crucial mediator of various cytokine signals from extracellular space down to the intracellular space and the nucleus, where transcription is done. Therefore JAK inhibitors are important as they can help to stop malicious cytokine signaling such as in the case of inflammatory diseases, autoimmune diseases, and certain neoplasms. The aim of this thesis is to review the known therapeutic effects of JAK inhibitors, including their mechanism of action due their molecular structure.