This interim report describes the progress of the nine ‘Trailblazer’ projects that received funds from the Social Enterprise Investment Fund in 2013 to investigate the feasibility of setting up Social Impact Bond (SIB) projects in health and social care in England. The findings discussed in this report are based on a literature review of the SIB literature and on documentary analysis and qualitative interviews with key informants involved in UK SIB development undertaken between May and November 2014.