This chapter will summarize the diagnosis, pathophysiology, societal impact, and management of chronic migraine and medication overuse headache. CM has a global prevalence ranging from 1.4% to 2.2% and significant individual and socioeconomic burden. CM is characterized by 15 or more headache days per month with at least 8 of these days having features of migraine with or without aura. The progression to CM from episodic migraine is complex. The mechanism of CM is not well understood, but it is thought to be associated with increased cortical excitability, central sensitization, as well as structural and functional brain changes on neuroimaging studies. For chronic migraine, evidence for pharmacological prophylaxis including onabotulinumtoxinA, topiramate, and the use of calcitonin gene-related peptide inhibitors will be explored. In addition, strategies to manage CM in the setting of analgesic overuse, a prominent risk factor for CM, will be discussed.