This chapter offers a multi-dimensional examination of criminal behaviour, connecting classical criminological theories with modern security challenges. Foundational frameworks such as Routine Activity Theory, Rational Choice Theory, and Strain Theory elucidate the mechanism by which crime is generated by situational opportunities, cost–benefit analyses, and socio-economic pressures. These insights remain pivotal for modern security contexts, including cybercrime and insider threats. The chapter also investigates the cognitive distortions, personality traits, and situational factors that influence the decisions of offenders. By integrating life-course criminology, it underscores the evolving trajectories of criminal careers.