The compartmentalization of psychological science and of the profession prevents the progress of the discipline. Compartmentalization is a collateral effect of the impressive scientific, methodological, and technical development of psychology, which has led to the emergence of specialized segments of knowledge and practice that unavoidably tend to progress separately from each other and weaken their reciprocal linkage. The work highlights the limits of compartmentalization and discusses motives that call for the unity of psychology. Three approaches to unification are outlined: I) the identification of the ultimate causal explanation; II) the progressive extension of the explicative capacity of specific theories; III) the building of a metatheoretical framework. Finally, the paper proposes the intervention as the criterion to compare the capacity of the three approaches to unity. According to this criterion, approaches can be validated by reason of their ability to enable professional psychology to address the current challenges that people and society have to face.