Reward system, from biological perspective, was developed as biological mechanism that controls behaviour and directs it towards events that facilitate survival of organism. Motivation has a big influence on it and some scientists believe that pleasure, as subjective quality, also takes part in it. Behaviorism comprises three fundamental principles of behaviour: positive reinforcement, negative reinforcement and punishment. Anatomically speaking, reward system is associated with mesolimbic-dopamine system. Most important structures of this system are ventral-tagmental area (VTA) and nucleus accumbens which are connected via dopaminergic pathway. Most common neurotransmitter included in reward system is dopamine. Reward system is also active in state of addiction, but in this case some other changes in secondary messenger system and gene expression in the cells will also occur. Withdrawal and tolerance are responses of compensatory system of the body to new conditions due to addiction. State of disturbed homeostasis, as a result of addiction, is called funcional neurotoxicity. There are many theories that try to describe state of addiction and one of the most popular is wanting-and-liking-theory. Today addiction imposes a great threat to society and many attempts have been made in order to reduce it and prevent it. Research of reward system and of drug effects on cellular level are welcome and may represent a way to solve the problem of addiction.