This book presents the major issues that criminology studies: why people break the law, the contexts in which they do it, and ways to reduce crime. In addition, it exposes in a clear and concise way how society reacts to crime and the functioning of the penal system (police officers, criminal justice system and punishment system). Criminology provides empirical knowledge based on criminological investigations. Therefore, in this second edition all the data referring to crime and the changes introduced by the victims movement have been updated. Variations in the penal system have also been updated, as a result of the implementation of the Organic Law for the Protection of Citizen Security, the reform of the criminal code of 2015, the impact of the economic crisis on crime rates and penitentiaries and the influence of European penology. In all the topics treated, a critical orientation is combined with an appreciative attitude towards the studied group, which describes its function, the complexity of its tasks, the contradictory demands to which its agents are subject and the multiple values ​​in conflict.