Bipolar disorder (BD) is a chronic brain disorder that causes dramatic changes in a person's mood and energy. People with BD undergo periods of extreme happiness and extreme sadness, known as episodes of mania and depression. A person undergoing an episode of mania often experiences euphoric moods, increased energy, and aggressive behaviors, while a person in a depressed state often experiences low moods, lack of energy, and feelings of sadness. Researchers have recently discovered that abnormalities in certain brain areas that control emotion are often present in those with BD. It is believed that the severe mood shifts in BD may cause alterations in normal brain activity, specifically in corticolimbic activation and connectivity. Current mood stabilizing medications used to treat people with BD may help normalize these brain activity abnormalities. This study will assess the effects of BD on brain activity using functional magnetic resonance imaging (fMRI) and will determine if treatment with lithium changes the brain activity in people with BD.