Chronic pain is a maladaptive condition which lasts longer than 3 months and accompanies with affective, cognitive and motivational disorders. It leads to decreased functions and deterioration in life quality, and requires a multimodal treatment. Nociceptive, neuropathic or mixed pain could be chronic. Chronic pain involves a series of molecular and cellular progressions. Prolonged inflammation, peripheral sensitization and central sensitization alter pain transmission and process. Therefore, these mechanisms that cause development of chronic pain lead to differences in the progress of pain signal reception. In this study, these mechanisms causing pain turn into a chronic state are reviewed.