Engage. Respond. Innovate. explores how library services in the UK could use an alternative methodology: a hackathon, as an opportunity to start to meet some of their wider organisational ambitions, to engage with the public, respond to evolving needs and innovate services. The report provides an introduction to the principles of a hackathon; details the benefits, limitations and considerations of the approach; presents the different ways in which public libraries and staff can be involved with hackathons using UK and international library examples; shares case studies including two UK services, who have adapted the hackathon methodology to deliver their own events, and concludes by summarising the learnings from across the programme and signposting a range of useful resources.