Team training is essential for building the knowledge, skills and attitudes that are required for effective teamwork. In the UK Emergency Services, one goal of team training has been to promote greater interoperability between different response agencies, however the efficacy of this training has not been tested. The goal of this research was to explore the perspectives of UK emergency responders to evaluate their experiences of interoperability training. We used a mixed-methods online survey methodology to evaluate current training. Quantitively, we found that participants rated both small- and large-scale live exercises, and in-person training as the most useful. E-learning was the only type of training where no participants rated it as extremely useful, with a median rating of slightly useful. Qualitatively, we asked participants to reflect on past experiences and think about ideals on what makes good interoperability training. We identified five themes that reflected the need for multi-agency training to be: (i) representative and realistic; (ii) focused on sharing perspectives and developing awareness of capabilities and challenges across teams; (iii) prioritised as a core part of the day-job for emergency responders; (iv) face to face rather than remote; and (v) a platform for building social relationships. Taken together, we recommend that future interoperability training be regular, interactive, practical, and social, to truly achieve behavioural change.