Supervision is a critical component of initial training and continuing professional development for applied psychology practitioners, and effective supervision is significantly related to the quality of the relationship between supervisor and supervisee. The core task of supervision is to engage in a relational process that provides containing and security, thus facilitating professional growth through reflection on experience. Two key arguments are proposed in this paper: (i) models that support theory-to-practice connections are essential for both supervisees and supervisors and (ii) models informed by psychological theory that place relating and reflecting at the heart of the supervisory process promote practitioner development and effective outcomes for clients. One model -- the Relational Model of Supervision for Applied Psychology Practice (RMSAPP) -- is outlined, and the systemic, psychodynamic and attachment lenses privileged within it are explored. The paper concludes with the strengths and challenges of the model, along with suggestions for future research directions.