This thesis explores psychological mechanisms underlying the acquisition, interpretation and exploitation of information in complex criminal enquiries. Detective work is conceptualised as problem-solving and the importance of sense-making is highlighted. A model of investigative sense-making is presented, grounded in social-cognitive psychological and criminological research and bringing together several theoretical concepts within one coherent framework. Two studies explored aspects of this framework. First, 42 UK police officers gave written responses to four crime-related vignettes. Content analysis of the answers showed how sense-making about what had occurred varied according to the vignettes and between participants. Building on this pilot, a simulated investigation method was developed and tested with 22 UK detectives. Qualitative content analysis of ‘think aloud’ transcripts (using the qualitative analysis package N-Vivo) focused on how participants made sense of the victim’s story, the characteristics of the offender and the plausibility of potential suspects. Participants spontaneously generated and tested multiple hypotheses about investigative information using mental simulation, tolerating high levels of uncertainty throughout the ‘investigation’ and paying particular attention to investigative opportunities. This research suggests that successful detectives need the ability to imagine multiple potential explanations for investigative data and the knowledge to identify the opportunities for action such data affords.