The intersection of women’s fashion and big business in the US has always been acompelling study across social strata. The ready-to-wear apparel industry thrives on creating a presumably original design that is then interpreted into copies. Called design piracy by some and the knock-off process by most in the industry, copying fashion designs is a firmly embedded business strategy that predates even the advent of women’s ready-to-wear in the late nineteenth century. Historically, some industry organizations and individual designers accepted and supported copying as crucial to the transmission of fashion; others strove to prevent the practice, arguing harms ranging from lost profits to the abuse of labor. Threaded through the complicated and fascinating history of US ready-to-wear fashion are more than eighty attempts to legislate for design protection, and countless efforts to stymie piracy through patents, trademarking, or industry self-regulation. The authors analyze legal and apparel industry documents; governmental reports; and their own primary research conducted in museums, archives, and special collections to shed light on arguments both for and against design piracy. A main focus is the Fashion Originators Guild of America (FOGA), one of the most successful industry organizations to attempt design protection. Knock It Off puts into perspective the conflicting interests that have always set fashion design apart from other creative works and continue to make the industry an endlessly perplexing and risky business.