The notion of subsidiary obligation, its relation to subsidiary liability, as well as certain types of civil obligations are considered in the present article. The concept of subsidiary obligation is one of the problems of Russian civil law. The Civil Code of the Russian Federation does not confirm the concept of subsidiary obligations. Currently, there are practically no studies dealing with the notion of subsidiary obligation. All the uses of the term "subsidiary" in the Civil Code are associated with the concept of "subsidiary liability". Thus in legal literature, it is understood as a form of subsidiary liability. The author criticizes the popular point of view on subsidiary obligation as a kind of obligation with the plurality of persons. Subjects in such obligations are parties of one legal relationship. They each must execute contracted duties in their own part or jointly or, in case of violation, bear responsibility. The subsidiary and the principal are the debtors of the same creditor but in two separate legal relations. Therefore, the nature of their responsibility does not allow taking them as co-debtors-they are independently responsible. In addition, shared obligations and joint and several obligations themselves may be subsidiary. Therefore it is impossible to combine all the three types of obligations in the same category. This is confirmed by the Civil Code the Russian Federation, which distributes the obligation of the plurality of persons and joint and several liability in Ch. 22 "Performance of obligations" and Ch. 25 "Liability for breach of obligations". Comparison of subsidiary obligation and accessory obligation is another issue considered in the present article. The analysis of the norms of the Civil Code has been performed in the given article. The ratio of subsidiary and accessory obligations has been suggested justified by the fact that accessory obligation also has "additional" nature, which is inherent to subsidiary obligation. Of all the ways to ensure obligation that have accessory properties, subsidiary character is solely the responsibility of the guarantor. Subsidiary obligation is not an accessory obligation, but as the accessory one, has security in terms of its economic function. The analysis of the concept resulted in the conclusion concerning subsidiary obligation as protective relationship and this way subsidiary liability is realized.