The terminology "multimedia security" gained a new popularity in the mid-90s with the rapid rise of digital watermarking in an attempt to combat piracy of copyrighted content. This milestone incarnates the mutation of content protection techniques from conventional cryptography to signal processing techniques. Today, multimedia security encompasses a much wider range of techniques such as multimedia encryption, content fingerprinting, anti-camcording, passive forensic analysis. These various techniques share a common feature, namely operating in the presence of an adversary who aims at interfering with the nominal behavior of the system. In this talk, we will survey 20 years of research in multimedia security and highlight enduring technical challenges that remain to be solved.