In 2009, the International Labor Office published a list of operational indicators to help countries to spot the signs of modern slavery and human trafficking. This list was derived through the use of the Delphi method and was one of the first lists of its kind to be created. Whilst the Delphi method may have been an appropriate method in 2009 to generate a list of operational indicators, due to a lack of data, the reliance on this evidence 10 years later remains a pressing issue. Not least because this set of operational indicators was widely taken up within the U.K. guidance. The use of these indicators across practice for several years has gone untested in the academic literature. The reliance upon this categorisation, over 10 years later, is no longer appropriate given the advances in systematised data collection. The present thesis aimed to conduct an empirical investigation into the operational indicators of modern slavery and human trafficking, as experienced by frontline professionals and NRM data. By utilizing a mixed methods approach, qualitative interviews with frontline professionals were used explore their experiences of operational indicators and the challenges to identifying and supporting victims, before being tested quantitatively. Results showed that frontline professionals found indicator lists helpful, valid, and valuable to their role in identifying victims and addressing barriers to identification. Qualitative findings also revealed that frontline professionals participate in the generation of novel indicators. Quantitative analysis confirmed frontline professionals’ experiences of indicator prevalence and trends. Future research should attempt to explore the differences in indicator prevalence across victims’ journey to recovery. Estimations based on indicator prevalence within post-trafficking support services, cannot be relied up to inform professionals of what they might expect to see on the frontline. Similarly, estimations of indicator prevalence at the NRM stage, should not be used to inform victim support needs, which may evolve overtime.