This is the second of three articles that examines the issue of adolescent substance use. The previous article provided an overview of different substances and how they are used. This article will focus on patterns of adolescent substance use, the harm it can cause and the best prevention methods available. The final article will focus on treatment of adolescent substance use. This article starts by providing a brief overview of adolescent development as it has significant implications for substance use related harms. The article will then go on to discuss the drug related harms, focusing on three broad areas: the biological or physical harms, the psychological, emotional, or mental health problems and, finally, the social issues to which substances can contribute to. It is probably worth mentioning that I am deliberately avoiding using the word cause for the simple reason that causality is hard to prove in real word scenarios. For instance, we know that alcohol can negatively affect mood and that the more someone drinks alcohol, the greater the risk of developing a mood problem. However, we also know that having a mood problem makes someone more likely to drink. Trying to determine which caused which (and it may be something else that caused both the drinking and the mood problem) in an individual case, is often an impossible question and somewhat academic in any case.