Therapeutic alliance describes what happens in the interactions and relationships between the client and the therapist. There is evidence from mental health literature that a therapeutic alliance can have a significant impact on outcomes. A negative alliance can lead to feelings of hopelessness and disempowerment, whilst a positive alliance can improve adherence, engagement, satisfaction and outcomes. There is no similar research in the field of aphasia, yet many speech and language therapists, and other clinicians, feel this is an important component of delivering rehabilitation. This paper describes the development of the Aphasia and Stroke Therapeutic Alliance Measure (A-STEM) using literature on therapeutic alliance, other measures from mental health, qualitative research with speech and language therapists and data generated from a Q-methodology with people with aphasia. The paper then provides an overview of the psychometric evaluation of the A-STEM with 35 participant dyads (people with aphasia and their speech and language therapists) across seven NHS trusts. Tests of reliability and validity were positive, but the authors recommend caution and suggest further large-scale investigations. Additionally, this preliminary investigation and tool did not capture the family experiences. Quantifying the therapeutic relationship and its impact will be a valuable component of future intervention studies and will lend further credence to investigations of fidelity and effectiveness in this area.