In the U.K., 270,705 adults were in contact with drug and alcohol treatment services between April 2019 and March 2020. Within the same time period, 118,995 individuals exited the treatment system, and just over a third (36%) left treatment without completing it. The latter includes individuals declining further treatment and unsuccessful transfers between services. The aim of this study was to explore the factors that affect drug and alcohol treatment uptake within a drug and alcohol service in North East England. A mixed-methods approach was adopted. The exploration of factors affecting treatment uptake was captured through a behavioural insights survey and 1:1 in-depth qualitative interviews with service users within one council area within the North East of England. There were 53 survey participants, and a further 15 participants took part in qualitative interviews. We triangulated data sources to report consistencies and discrepancies in the data. Findings show that treatment services aiming to reduce missed appointments and increase retention rates need to implement several strategies. Consistently distributing appointment cards, using text message reminders, displaying a timetable presenting all treatment options, and displaying information in a format to ensure it is accessible to individuals with lower health literacy and reducing wait times for appointments will all improve appointment attendance.