In the article there are compared and generalized the opinions of various authors on the complex of conditions, which must be met by the sources of competitive advantage for the latter to be sustainable. Research is founded on the views of the resource approach, according to which not all resources possess the same potential to be sources of sustainable competitive advantage. The requirement for the resource to be difficult to imitate affirms itself as a key condition for the sustainability of the advantage. There are also mentioned examples of resources that are difficult to imitate, for which various studies (including the author’s) prove a positive connection with the achieved advantage of the organization in the long run.