In recent years, the incidence of lung cancer (LC) has been increasing throughout the world and is the most common type of cancer in all regions of the world, occurring more frequently in men than in women. Paraneoplastic syndromes (PNS) refer to clinical conditions that develop in relation to tumors, without physical effects of the primary or metastatic tumors. The development of PNS is not associated with the size of the primary tumor or the extent of metastases. It is usually seen in small-cell lung cancer (SCLC) as well as other types of lung cancer. PNS developed in almost 1 in 10 patients with lung cancer and it may be an indicator for the diagnosis of lung cancer and it can be seen during later stages of cancer or at the time of cancer recurrence. Accordingly, the identification of these syndromes can be helpful in the early diagnosis of occult cancers, allowing timely treatment. PNS decreases the quality of life of the patients with cancer and thus requires specific treatment. Moreover, these conditions can be used as a marker of cancer activity and can predict prognosis. In this section, a detailed description of PNS is provided.