Research into the effectiveness of counselling and psychotherapy has great practical significance, as a means of collecting evidence that may potentially enhance the quality of services delivered to users. In recent years, the evidence base for counselling and psychotherapy has increasingly relied on data derived from self-report questionnaires completed by clients, with relatively little attention being paid to therapists' evaluations of outcomes. To make full use of therapist estimates of outcome, it is important to develop an understanding of the processes and criteria that therapists employ when making such judgements. However, little is known about the evaluation strategies used by therapists in their everyday practice. The aim of the present study was to explore the implicit and informal construction of outcome evaluation by experienced practitioners. Person-centred therapists were interviewed about their approach to evaluation. The interview data were analysed using a grounded theory approach. These practitioners reported that they engaged in a process of evaluation based on a range of different sources of evidence, which was then “weighed up”. Evaluation was a continuous activity that was embedded in the counselling process itself rather than arising from discrete measurements carried out at particular times. The findings of this study suggest that practitioners may possess a sensitivity to the complexity of outcome that is missing in much current research. Implications for training, research and practice are discussed.