The article examines the problem of the legality of the transaction in civil and tax disputes. The existence of opposite approaches to this issue is revealed. The causes of this problem are identified at the levels of legislation, law enforcement practice of courts and tax authorities, research and training. Proposals are being made to ensure a uniform approach to determining the legality of the transaction. They are associated with the improvement of the systems nature of Russian law, the solution of the question of the relationship between civil and tax law, a systems approach to the adoption of laws, changes in the law enforcement system, comprehensive scientific research and the formation of a systems view in the training of legal and economic specialists.