This chapter describes the main psychological theories that have offered a means to understanding criminal behaviour and the offender. Each theory provides the forensic psychologist with a research methodology and a therapeutic approach. Evolutionary psychology is considered as an overarching approach that offers a framework into which the other areas of psychology can be weaved. Biopsychology also offers physiological explanations and focuses on brain development through a gene-environment interaction. The cognitive and behavioural psychology offers forensic psychologists a good understanding of the reasoning mind and behavioural output and also provides forensic psychologists with a powerful antidote to some of the more deviant criminal behaviours that exist. Both environmental and situational approaches to understanding criminality contributes interesting ideas to the mix of explaining criminality. The concept of victim precipitation is controversial and perhaps viewed as being politically incorrect, but recent understanding of serial killers and how they target their victims reopens the idea of passive victim precipitation.