The US Federal Trade Commission requires that you disclose use of affiliate links. More information and tools are available.
Sovrn //Commerce, formerly VigLink, works in your existing content; all links that are inserted look as if you inserted them yourself and mousing over a link will always reveal the destination. There are no intrusive pop-ups, ads or double underlines. So, most users do not notice on their own, but we highly recommend you disclose it.
Even if you are not subject to FTC jurisdiction, we strongly recommend you disclose. Our customers have found that readers are completely understanding if they are informed and can be upset if they are surprised. Informing your users will give those who desire it the opportunity to permanently opt-out. Although Sovrn //Commerce is not intrusive, it is easily detectable by savvy users.