Background: The impact of new psychoactive substances (NPS), also known as legal highs, on UK drug markets is a serious health concern. Young people are thought to be especially at risk of underestimating dangers of NPS use due to the association of the term legal highs with perceptions of safety. However, in the UK, few studies have focused exclusively on young people’s perceptions of NPS and there is no evidence on the effectiveness of interventions addressing young people’s NPS use. Aim: This thesis aims to inform the development of interventions that seek to address young people’s use of NPS. The research explores young people’s beliefs, reasons and motives for using and not using NPS with the aim of linking their beliefs about NPS with theoretical frameworks suitable for use in interventions. As the research took place across the period when the Psychoactive Substances Act (PS Act) was implemented, the extent to which its implementation affected young people’s perceptions about NPS is also examined. Methods: Three qualitative studies were conducted to explore young people’s perceptions of NPS, the acceptability of potential intervention approaches, and to compare differences in their beliefs about NPS before and after the introduction of the PS Act. Results: Findings showed that young people’s NPS use and non-use were related to multiple motivations that are well described by extended versions of the Theory of Planned Behaviour. Pre-and post-ban comparisons indicated that young people’s perceptions of NPS appeared to be linked with the increased visibility of problematic NPS use in marginalised populations. Conclusion: The thesis concludes that social cognition theories offer potential for supporting the development of effective interventions, when used as part of a wider prevention strategy that remains sensitive to social inequalities and adopts a systems perspective which takes into account wider contextual factors.