This thesis presents a theoretically informed social history of the New Zealand Needle Exchange Programme (NEP) which has operated since 1988. Close attention is paid to how this 'harm reduction' programme demonstrates a pattern of 'peer-professional' hybridity - a form of quasiprofessionalism developed by injecting drug user (IDU) peers who began operating private needle exchanges funded by both illicit clients and state agencies. In this hybrid mechanism, the personal distrust required to pursue 'criminal' motivations has been connected, through the vulnerable yet influential intermediaries of peers and syringes, to the trust required to 'empower' the health of marginalised IDU communities. This research has drawn on immersed participant experience and on accounts from archival documents, supported by interviews. A reworking of actor-network methodologies has provided a core analytical approach to tracing the critical moments and boundary-shifts in the development and realignments of the NEP's hybrid heterogeneous assemblages. The assembling and reassembling has entangled policy goals, technologies, historical reviews, stigma, laws, logics, logistic systems, narratives, organisations, sterile and bloody syringes, monitoring systems, and professional occupations. IDU, health policy officials, peerprofessionals, managers, politicians, HIV/AIDS community organisers, and medical professionals have prevented HIV transmission by altering key strategic connections and alignments within this active network, while pursuing their public-private interests. The peer-professionals have publicly represented IDU, have advocated professionally for inclusive rather than exclusive public health provisions, while guaranteeing that the monitoring of syringes by state agencies would not harm IDU. The difficulties in shaping and stabilising the NEP have illustrated the 'messy reality' of its institutional and policy environment, yet have also led to highly successful and sustainable health promotion work.