This chapter investigates the complex mechanisms and clinical manifestations of mood dysregulation, focusing primarily on major depressive disorder (MDD) and bipolar disorder. This chapter outlines the core symptoms and diagnostic criteria for these conditions and other related affective disorders. It then details the various animal models used to study the neurobiology of mood, including stress-induced learned helplessness and the sucrose preference test, which are vital for understanding the neuromechanisms of mood disorders. This chapter is dedicated to mapping the neurocircuitry involved in mood dysregulation, specifically the limbic-cortical-striatal-pallidal-thalamic (LCSPT) pathway and the default mode network (DMN), as well as the neurochemical systems implicated in mood disorders, including serotonin, norepinephrine, dopamine, glutamate, and GABA. The role of neuroinflammation on mood dysregulation is also covered. This chapter concludes by discussing various forms of affective impulsivity, such as urgency and self-injurious behavior.