Pfaff v. Wells Electronics, Inc., 525 U.S. 55 (1998), was a decision by the Supreme Court of the United States that determined what constituted being "on sale" for the purposes of barring the grant of a patent for an invention.

[1] In November 1980, the plaintiff, engineer Wayne Pfaff, was asked by Texas Instruments to design for them a socket for the mounting and removal of semiconductor chip carriers.

Pfaff could have patented his idea based on the drawings that he had shown to Texas Instruments, because they were complete enough to allow another engineer to build the invention from the designs.

Furthermore, if an invention could be sold before it was reduced to practice without any consequence, then inventors would be able to evade the time limits placed on the patent itself by simply delaying the construction of a working model.

This would, in turn, delay new inventions from reaching the public, undermining the primary goal of the patent system, "[t]o promote the Progress of Science and useful Arts..." Based on these considerations, the Court concluded that the "on sale" bar applies if two conditions are met: Because these conditions were satisfied, the Court held that Pfaff's patent was invalid.