Trans fats became part of the American food system due to a complex interplay among activism, industrial technology, and nutritional science. Some manufacturers began using partially hydrogenated oils, which contain trans fats, in the early twentieth century. Medical authorities began framing saturated fats as unhealthy in the 1950s. In the 1980s, activist organizations, including the Center for Science in the Public Interest, condemned food corporations’ use of saturated fats and endorsed trans fats as an acceptable alternative. Nearly all targeted corporations responded by replacing saturated fats with trans fats, which fit easily into their existing products. Trans fats thus became the perfect solution to the political problem of saturated fats and to the technical problem of what to use in their place. Activists helped precipitate technological change, but by 1994, trans fats were no longer regarded as a solution. Instead, they became regarded as a new nutritional problem.