The aim of this article is to empirically clarify factors and conditions of expertise. In addition to the core concept of expertise as excellence, a second factor needs to be taken into account: professionalism, or professional engagement. This hypothesis was tested and confirmed using data obtained from a survey on Swiss environmental professionals (n = 3514). The empirical test was based on factor analyses (Study 1) and a structural equation model (Study 2). The expertise scale consisted of nine items for self-assessed perceived expertise amongst professional peers ([alpha] = 0.78). Excellence turned out to be a logarithmic function of years of practice (Study 3). Professionalism prevailed in domains where standards for best practices still needed to be established (Study 4). (Contains 5 tables, 2 figures and 3 notes.)