Opinions on the issue of incompatibility are prepared by the Bureau of Research of the Chancellery of the Sejm, directly or through its agency, within the framework of consultative tasks specified in the rules of procedure of the Sejm. Usually, they are produced in the first months of the term of office of the Sejm and are connected with the exercise by the Marshal [Speaker] of the Sejm of his power to pronounce the expiry of the mandate due to the holding by the Deputy of a position subject to incompatibility. The author attempts to sum up the previous results of work of the Bureau of Research of the Chancellery of the Sejm in this field and to indicate main legal problems addressed in the opinions concerning the issue. From an analysis of these opinions it follows that major difficulties relate, firstly, to relations between the notion of 'employment' used in Article 103(1) 'in fine' of the Constitution and 'performance of work on the basis of labour relationship' used in Article 30(1) of the Act on the Exercise of the Mandate of a Deputy or Senator and, secondly, the interpretation of the notions of state and local government administration in Article 30(1) of the Act on the Exercise of the Mandate of a Deputy or Senator. It turns out that the authors of opinions present various solutions of the above-mentioned problems and often their opinions contain totally different conclusions. Taking into consideration their doubts about interpretation of these notions, it is possible to identify prevailing views. A survey of the opinions leads to the conclusion that arising doubts tend to be resolved in favour of incompatibility. Such judgements, as a rule, are justified by the aim of the institution, its guarantee function. The connection between incompatibility and the principle of separation and balance of powers is also pointed out. The authors of the opinions who present different (narrower) approaches do not agree as to the criterion which justifies a more narrow of the notions in the normative context. The results of the survey confirm critical views of the theory concerning the existing regulation on incompatibility of the mandate of a Deputy. The notions and wording used are ambiguous to the extent that it endangers the principle of legal certainty. The proposal to amend the Act on the Exercise of the Mandate of a Deputy or Senator, offered by legal theorists is reasonable.