The paper is devoted to such an actual problem in the field of audit activity as professional judgment and an auditor's opinion. A proper clarification of the logical content of these concepts significantly affects the effectiveness of the audit according to requirements of International Standards on Auditing (ISA). To date, in scientific publications the various approaches to the interpretation of the given concepts are expressed. A significant part of the authors is limited to general definitions and only some of them make attempts to substantiate their research with the laws and categories of logic. At the same time, native authors in every possible way avoid a full-fledged logical definition of content of the concept "auditor's opinion". The main reason is that there is an unsuccessful translation from English terminology into Ukrainian. In some cases, the term "opinion" has been translated as "opinion", and in others it has been called "conclusion". Therefore, if the term "opinion" is preferred, it turns out that the audit begins with an auditor's opinion and ends with it. An auditor's opinion fully encompasses such forms of logical thinking as concepts, judgments and inferences. The paper not only shows the misinterpretation of the concepts of "professional judgment" and "auditor's opinion" in a number of scientific studies. Also it is proposed the own vision of the cognition process in the auditing field based on the laws of general and dialectical logic. A related scheme of an auditor's professional judgment is given. From this scheme it can be seen that the opinion of a professionally competent auditor from the very beginning of the audit is being transformed through rationale into the concepts and requirements of the ISA. Further, an auditor's rationale is being transformed into judgments and conclusions. Based on them, as a result of proving or proving and planning, the specific conclusions of an auditor are formed as a report. The paper states that an auditor's professional judgment, like the professional judgment of other specialists, is subordinated to the general laws of logic. Its specificity lies in the fact that it is carried out with professional skepticism in conditions of uncertainty and risk and is subject to the requirements of legislation and ISA.