Depression is a medical condition characterized by feeling sad even when good things happen, having low energy and motivation, and sometimes even experiencing suicidal thoughts. Bipolar II Disorder is an illness in which periods of depression alternate with periods of abnormally elevated mood, energy and activity, referred to as hypomania. After Major Depressive Disorder, Bipolar II Disorder is the most common cause of depression. Unfortunately, antidepressant medications, used alone, do not work as well in treating Bipolar depression as they do in treating other kinds of depression. Lamotrigine is a medication which studies show is effective in treating Bipolar depression. The investigators will determine if lamotrigine works best to treat Bipolar II depression if it is used alone, or if it is taken with an antidepressant. In the first part of our investigation, people with Bipolar II depression who have not responded to an antidepressant will either add lamotrigine to their antidepressant, or will stop the antidepressant and take lamotrigine alone. They will see the study doctor for 6 visits over 8 weeks, and will answer questions about their depressive symptoms and their overall health. The purpose of this study phase is to determine which treatment works best to treat active Bipolar depression. In the second part of the study, people who have responded to their assigned treatment may continue to receive it for another 44 weeks. They will see the study doctor monthly, and will answer similar questions about their health. Participants will also receive a physical examination and get a blood test three times during the study. The purpose of the second phase is to ascertain which treatment is best at preventing relapses of depression. The investigators hypothesize that people who take Lamotrigine plus an antidepressant will recover from their depression more completely, have a longer period of wellness, and have better quality of life compared to those taking Lamotrigine alone.