This thesis is longitudinal study of two interrelated phenomena, the Single [Legal] Undertaking of the WTO and the accession practice of the Organization. It concludes that the Single [Legal] Undertaking, having emerged during the Uruguay Round as a package of rights and obligations aimed at unifying legal status of WTO Members following the fragmentation during the GATT era, is itself subject to limitations, maintaining its centrality only to a limited extent. As confirmed by the WTO DSM, accession terms can deviate from the Single [Legal] Undertaking and remain enforceable, unless restricting rights of other WTO Members. Such terms often pave the way to pluri- or multilateralization of the solutions tested through accessions. Currently, systematic deviations introduced by accession commitments mostly impact trade in goods, framework for making and enforcing policies and transparency domains, permanently or temporarily. Meanwhile, certain aspects of WTO accessions practice and outcomes reinforce the Single [Legal] Undertaking.