The relationship between the probation practitioner and the probationer is central to probation work. That is, for both parties, ‘the relationship’ is where the majority of “the work” is undertaken if the end goal of delivering better desistance outcomes is to be realised. Crucially, practitioners’ hold that probationers will not participate in probation work with practitioners unless they trust them. So, trust seems to be doing a lot of work in the probation relationship, but how practitioners mobilise, negotiate, and develop trust, or not, with probationers remains poorly understood, and too often neglected as an integral part of successful probation practice. As such this doctoral study examined the relational practice journey of the practitioner and the probationer, specifically considering which factors are facilitative of, and contribute to, the formulation of trust, and conversely, how trust building can be eroded or undermined in the probation relationship. To achieve these aims this study utilised a qualitative longitudinal case study approach to analyse the probation relationship. It found that practitioner approaches to relational and trust development differed, and that there are a number of different types of probation relationship. It also found that some probation processes, as well as practitioner approaches act as a barrier at crucial points in relational and trust development but, that when relationships and trust are broken, they can still be repaired. This has a number of implications for policymakers at the probation process, practice, and relational levels, as well as public protection, probation, and desistance outcomes.