Since long time mental health professionals and the patients suffering from Major Depressive Disorder are looking for some diagnostic test and severity assessment tool which measure the suffering of patient objectively or with lesser bias. Patient themselves want to undergo some test beside psychometric tests to have more faith in diagnosis and hereby more compliant for treatment. This book provides the answer of this longstanding question in form of measurement of P300 wave in depressive patients. In our study the clinical sample consisted of 30 consecutive subjects suffering from major depressive disorder and the statistical data at the time of diagnosis had been taken from previous study and then using same parameters, these patients evaluated for their P300 latency and HAM-D score after 3 and 6 months of the treatment. The present study confirms that P300 latency is significantly prolonged or delayed in the patients of the Major depressive disorder over the period of time. This study also confirms that P300 latency prolongation is directly proportional to the severity of the depression. This book will work as a valuable guide to Psychiatrist, Neuropsychologist and Neurologist.