Human Trafficking (HT) has serious social and economic implications for both society and the victims of the crime. Despite being one of the most complex crimes to detect and investigate, multi-agency collaboration can often underpin effective investigations. There remains, however, a scarce evidence-based knowledge concerning the investigation of HT and police collaboration with partner agencies. The present study examines police collaborations in England and Wales when investigating HT, providing empirical knowledge on (i) the types of support police officers usually require from other agencies when investigating HT crimes; (ii) the agencies with whom they usually collaborate; and (iii) the types of support agencies can provide. The study uses the Repertory Grid Technique to gather and analyse data from 28 investigators from nineteen police units in England and Wales investigating trafficking crimes. Data from the individual grids was analysed through content and descriptive analysis. A median grid was created and analysed through principal component analysis and hierarchical cluster analysis. The study identified that police officers need to collaborate with partner agencies when working with victims, planning and assisting police operations/strategies, building the criminal intelligence picture and obtaining information about premises/companies and individuals. Findings also reveal that the police believe they need to rely upon several key agencies for each category of support. Findings evidence the need for police collaboration with partner agencies to secure victim-centred and intelligence-led investigations.