Abstract: This article defines the concept of “professional task” and expounds the psychological structure of the process of solving a professional task by a subject of activity. Professional task means a unit of content of professional activity of a specialist in a particular area. The process of solving a professional task represents activity aimed at forming efficient model of solution on the bases of previous experience, existing algorithms and heuristics, in order to find functional and specific solution, as well as achievement of a goal under the set circumstances common for this type of activity. The methodological framework consists of the theoretical analysis, generalization and systematization of approaches towards determination of the essence and psychological structure of the task-solving process conducted by Russian and foreign researchers and practitioners. Analysis of the works of Russian and foreign authors devoted to solution of professional tasks allowed formulating theoretical grounds for diagnostics of success rate of task solving by a subject of professional activity. This article outlines the positions applicable for subsequent research of the process of solving professional tasks by a subject of activity with consideration of objective and subjective sides of the task.