The consultation market is shared by a number of professional fields, including psychology, engineering, management, finance, education, and computer science. Consultants are usually experts in a particular economic sector, area of organizational operations or field of knowledge. Organizational consulting is a process that is being adopted by more and more professional practitioners, particularly organizational psychologists. However, despite the ubiquity of the practice, there has been little applied research on the subject. The literature leaves consultants without any empirical references or concrete tools to meet the challenge of evaluating and improving the effectiveness of their own practice. In response to this problem, in-depth exploratory interviews were held with 15 experienced consultants. We seek to explore and document the general and operational terms of a concept that belongs to one area of the practice - that is, consultants’ evaluation of the effectiveness of their own consulting process. This implies having an understanding of their notions of effectiveness and the criteria they apply, which are most plausibly based on concrete examples in their professional practice and the reflections and conceptualizations these have elicited over the years. Analysis of the substantive transcripts of these interviews revealed a number of new findings. The consultants described a limited number of criteria related to effectiveness as an outcome, as well as to intervention as a process. They confirmed that there is more than one criterion to consider and that some measurements are subjective while others are objective. The study also helped clarify how these criteria are used and suggested a number of criteria that could be used. The opinions of these expert consultants converge to reveal six effectiveness criteria that allow consultants to measure and ascertain their own effectiveness and, if necessary, reorient the mandate. These criteria also serve as reference points for researchers who want to link the activities of the consultancy process with its outcomes. In addition to helping us better understand the criteria employed by practitioners, a comparison of this study’s results and the literature on effective consulting provides an opportunity to review the definition of this concept.