At least for our agency, the one service that clients request more than any other is surveillance. Some refer to it as "tailing", "following", "shadowing", "staking out", etc., but the idea is the same no matter what you call it -- we discreetly observe, document (photo and video) and report on what the target individual is doing, where he/she is going and with whom he/she is meeting. Of course, we have a number of sexier, high-tech approaches that we also use in addition to or in conjunction with surveillance, but nothing beats actual photo/video evidence and a direct, eyewitness account from an experienced surveillance professional.