This project aims to develop effective harm-reduction messages and strategies for young adult illicit stimulant users. It focuses on identifying threshold moments in the lives of users of amphetamine-type stimulant (ATS) such as ecstasy [MDMA] and methamphetamine. Data for this project comes from a broader study, the Natural History Study of Drug Use - a longitudinal study that followed a population-based cohort of young adult ATS users for four and a half years. In-depth qualitative interviews were conducted at the start of the study with these users. This project analysed the baseline interview data, as well as that from four follow-up waves over the study's four and a half years. This document compares patterns of ATS and other substance use between well-managed and at-risk ATS users. It also explores harm-reduction practices among ATS users and discusses proposals for effective harm-reduction messages and strategies for young adult ATS users.