Over the past years, digital images have been widely used in the Internet and many other applications. As image processing techniques are developing rapidly, tampering digital images without leaving any obvious traces becomes easier and easier. Passive image forensics is a technology of detecting image authenticity and source without relying on any pre-extraction or pre-embedded information. According to the forensics characteristics used by image authentication, the techniques of passive image forensics for authenticity detection are divided into three categories: techniques based on the traces left by the process of image forgery, techniques based on the consistency of imaging equipment, and techniques based on the statistical characteristics of natural images. The basic characteristics of typical methods are summarized in detail for each category. Passive image forensics has become a hot topic with broad prospect in multimedia security.