There are several different types and sub-types of depressive illness just as Heart disease may present in different ways. Three of the more common forms are:

 
• Major Depressive Disorder (MDD) - defined as a depressed mood or loss of interest and pleasure in almost all activities for at least a two week period. Several other symptoms must also be present. These include sleep disturbances, appetite disturbances, changes in energy levels, difficulties with thinking and concentration and sexual difficulties. These symptoms interfere with usual behaviour and functioning.  
• Dysthymia - many of the same symptoms as those for a MDD are present but they tend to be less severe and interfere less with immediate functioning. They are, however, chronic and may continue for years so that the sufferer seldom feels really happy and that they are enjoying life. Due to the long-term impairment of functioning, many do not realise their full potential. Dysthymia can therefore have severe long-term consequences and can be severly disabling.  
• Bipolar Disorder - This used to be called manic depression. This is much less common than the two previously mentioned depressive disorders and only 2% of the population is affected over a lifetime. Males and females are affected equally. This type of depressive disorder involves episodes of depression and episodes of mania/euphoria. The switches between these two states may be fairly sudden and dramatic but are more commonly gradual in onset. Both mood states may co-exist - mixed bipolar disorder. During episodes of mania judgement is often impaired and this can result in socially embarrassing behaviour, sexual indiscretions, excessive spending and unwise business decisions. Bipolar disorder tends to be a chronic, recurring condition and is generally considered to have a poorer long-term outcome than Major Depressive Disorder.

Other types of depressive illness include:

 
• Minor Depressive Disorder (same duration but less severe symptoms than MDD)  
• Recurrent Brief Depressive Disorder (same symptoms as MDD but episodes last less than two weeks)  
• Premenstrual Dysphoric Disorder (experiencing for at least one year depressive symptoms that occur during the last week before menstruation)  
• Post-partum Depresssion (depression following childbirth that is more severe and of longer duration than transient “Baby Blues”)

Depressive Disorders may also be related to drug and alcohol abuse as well as to prescription drug usage (Substance Induced Mood Disorders) and to medical illnesses (Mood Disorder Due to a General Medical Condition).