Indicator displacement assays (IDAs) are now a popular method for converting most any synthetic receptor into an optical sensor. In this review many such assays are highlighted, along with biological counterparts. The focus is upon colorimetric, fluorescent, and metal containing IDAs. The power of the method can be readily appreciated by the large diversity of analytes that have been targeted with this technique. It is clear that the method is now well accepted and will continue to be one of many methods used to create optical detection methods from synthetic receptors.