Patent infringement cases involving software products can raise a number of unique challenges. One such challenge arises in connection with 35 U.S.C. Section 273, which provides a defense to patent infringement based on prior commercial use of a product or system. Evidence of prior commercial use of a device can be fairly straightforward; but evidence of prior commercial use of software can raise different and complex challenges in connection with source code, version control, and related issues. In an increasingly software-based world, companies should be mindful of these challenges, and best position themselves to raise—as needed—this Section 273 defense.