Bipolar disorder (BD), previously known as manic depression, is a mental disorder characterized by periods of depression and of abnormally elevated mood that each last from days to weeks, and in some cases months. If the elevated mood is severe or associated with psychosis, it is called mania; if it does not significantly affect functioning, it is called hypomania. During mania, an individual behaves or feels abnormally energetic, happy, or irritable, and often makes impulsive decisions with little regard for the consequences. There is usually sleep disturbance during manic phases. During periods of depression, the individual may experience crying, have a negative outlook, and demonstrate poor eye contact. Over a period of 20 years, 6% of those with BD died by suicide, with about one-third attempting suicide in their lifetime. Among those with BD, 40–50% overall and 78% of adolescents engaged in self-harm.

While the causes of this mood disorder are not clearly understood, genetic and environmental factors are thought to play a role. Genetic factors may account for up to 70–90% of the risk of developing BD. Environmental risks include a history of child abuse and long-term stress. The condition is classified as bipolar I disorder if there has been at least one manic episode, with or without depressive episodes, and as bipolar II disorder if there has been at least one hypomanic episode (but no full manic episodes) and one major depressive episode. It is classified as cyclothymia if there are hypomanic episodes with periods of depression that do not meet the criteria for major depressive episodes. If these symptoms are due to drugs or medical problems, they are not diagnosed as BD.

Mood stabilizers, particularly lithium, and anticonvulsants, such as lamotrigine and valproate, as well as atypical antipsychotics are the mainstay of long-term pharmacologic relapse prevention. Antipsychotics are used for acute manic episodes or when mood stabilizers are ineffective or not tolerated, with long-acting injectables available for patients with adherence issues. There is evidence that psychotherapy improves the course of BD. Use of antidepressants in depressive episodes is controversial: they can be effective but certain classes of antidepressants increase the risk of mania. The treatment of depressive episodes, therefore, is often difficult. Electroconvulsive therapy (ECT) is effective in acute manic and depressive episodes, especially with psychosis or catatonia. Admission to a psychiatric hospital may be required if someone is a risk to themselves or others; involuntary treatment is sometimes necessary if someone refuses treatment.

BD occurs in approximately 2% of the population. Symptoms most commonly begin between 20-25 years old; an earlier onset is associated with a worse prognosis. Around 30% of people with BD have financial, social or work-related problems due to the condition. BD is among the top 20 causes of disability and leads to substantial societal costs. Due to lifestyle consequences and medication side effects, risk of death from natural causes, such as coronary artery disease, in people with BD is twice the average.

Late adolescence and early adulthood are peak years for the onset of bipolar disorder. The condition is characterized by intermittent episodes of mania, commonly (but not in every patient) alternating with bouts of depression, with an absence of symptoms in between. During these episodes, people with bipolar disorder exhibit disruptions in normal mood, psychomotor activity (the level of physical activity that is influenced by mood)—e.g. constant fidgeting during mania or slowed movements during depression—circadian rhythm and cognition. Mania can present with varying levels of mood disturbance, ranging from euphoria, which is associated with "classic mania", to dysphoria and irritability. Psychotic symptoms such as delusions or hallucinations may occur in both manic and depressive episodes; their content and nature are consistent with the person's prevailing mood. In some people with bipolar disorder, depressive symptoms predominate, and the episodes of mania are always the more subdued hypomania type.

According to the DSM-5 criteria, mania is distinguished from hypomania by the duration: hypomania is present if elevated mood symptoms persist for at least four consecutive days, while mania is present if such symptoms persist for more than a week. Unlike mania, hypomania is not always associated with impaired functioning. The biological mechanisms responsible for switching from a manic or hypomanic episode to a depressive episode, or vice versa, remain poorly understood.

Also known as a manic episode, mania is a distinct period of at least one week of elevated or irritable mood, which can range from euphoria to delirium. The core symptom of mania involves an increase in energy of psychomotor activity. Mania can also present with increased self-esteem or grandiosity, racing thoughts, pressured speech that is difficult to interrupt, decreased need for sleep, disinhibited social behavior, increased goal-oriented activities and impaired judgement, which can lead to exhibition of behaviors characterized as impulsive or high-risk, such as hypersexuality or excessive spending. To fit the definition of a manic episode, these behaviors must impair the individual's ability to socialize or work. If untreated, a manic episode usually lasts three to six months.

In severe manic episodes, a person can experience psychotic symptoms, where thought content is affected along with mood. They may feel unstoppable, persecuted, or as if they have a special relationship with God, a great mission to accomplish, or other grandiose or delusional ideas. This may lead to violent behavior and, sometimes, hospitalization in an inpatient psychiatric hospital. The severity of manic symptoms can be measured by rating scales such as the Young Mania Rating Scale, though questions remain about the reliability of these scales.