Trade secrets and patents have a thin line of difference, indeed, and it is particularly for the integrity of trade secrets that prior user rights are very important. It is not possible and practicable to obtain patents on all patentable albeit marginal inventions and it would be much too costly. Being 'first,' however, does not automatically entitle an inventor to a patent. In certain instances a subsequent independent inventor may receive a patent. In other instances, the first inventor may receive the patent after a subsequent independent inventor has already commercially used, or taken substantial steps to commercially use, the invention before the filing date of the first inventor/patentee. It is in these situations that the concept of prior user rights is relevant. These rights would provide a limited defense for any party who independently develops or uses the subsequently patented invention, in good faith, before the patentee's filing date, who continues such development or use after the patent issues, and who is sued by the patentee for infringement. In this situation, the party (the prior user) could claim a prior user right and legally continue to exploit the invention in a manner that normally would constitute infringement. This article addresses the rationales for the adoption of prior user rights in the United States' first-to-invent patent system with a comparative study in the Indian legal scenario.