The possession and viewing of child sexual abuse material (CSAM) is a worldwide problem and it has proven difficult to address. Those who offend through the use of CSAM seem to have different characteristics to other types of sexual offenders, while the online environment appears to be enabling this type of offending behaviour. Existing theories of CSAM offending often approach the issue from the perspective of general sexual offending or tend to focus on the offender’s internal characteristics and long-term vulnerabilities. This might underestimate the importance of the offending process itself, psychological factors that are proximal to the offence, or interactions between the individual and the physical, digital or social environment. Currently, the most dominant response to CSAM possession is the apprehension of those who offend, although it is generally recognised that societies cannot ‘arrest their way out’ of the problem. Situational crime prevention, the social - ecological theory, as well as deterrence and desistance theories could extend current understanding of how CSAM possession and viewing could be prevented. The first aim of the present thesis was to investigate the process of CSAM viewing and possession, and desistance, with an emphasis on contextual factors that are proximal to the offence. The second aim was to utilise the above findings in understanding the challenges and the opportunities in preventing the viewing and possession of CSAM. These aims were achieved through a grounded theory study, which investigated: (1) the process of CSAM viewing and possession, including proximal and immediate contributing factors, (2) the interactions between the individual and the social environment while offending and desisting, and (3) help-seeking behaviour before and after arrest. Fifty semi-structured interviews were conducted, with a community sample of 47 individuals arrested and charged with CSAM possession offences. The study analysis led to the construction of a process theory of CSAM possession and viewing, which highlighted three different pathways to accessing CSAM: (1) through the viewing of legal pornography, (2) through online interactions with others, (3) direct CSAM access (without prior viewing of legal pornography). Most participants would initially mention stumbling on CSAM, but, over time, the process would become increasingly purposeful, and their access skills would significantly improve. Interruptions of the offending activity and re-engagements were frequent, while affective states and permissive beliefs appeared to play a role in the initiation, continuation and interruption of offending. The theory further illustrated the participants’ CSAM-related social interactions and help-seeking behaviour before and after arrest. Prior to their arrest, many participants would engage in CSAM-related online interactions, while keeping their CSAM-related activity secret. No one around them would be aware of it and they would rarely make any related help-seeking attempts. After arrest, the participants would experience the consequences of their offending, have a sense of being held accountable for it, and would be concerned or worried about who else finds out. After their arrest, all had sought professional help or support to address their offending and other related problems, partly as a way of demonstrating remorse in court, although some would also seek and receive support by partners, spouses, family members or close friends in desisting from offending. The thesis concludes with a discussion of the study findings, which generally appear to confirm previous research in the area. At the same time, they extend some of the existing knowledge further, by highlighting the role of affect throughout the offending process, as well as the role of crime controllers and social relationships in offending and desisting. A crime script analysis of CSAM possession and viewing is also presented, which is subsequently used as a guide to map out strategies for prevention.