Many information systems that provide useful services to people are connected to the Internet for convenience and efficiency. However, improper accessibility might make the systems susceptible to a variety of attacks. Although existing security solutions such as an intrusion detection system, intrusion prevention system, and firewalls have been designed to protect against such attacks, it is impossible to block all kinds of attacks. Furthermore, most of the proposed solutions require information about attacks for efficient prevention. Research on intrusion-tolerant systems (ITSs) have been conducted in order to continue providing proper services in threatening environments. The purpose of an ITS is to survive against every intrusion, rather than to prevent them. In this paper, previous studies on ITS are introduced and classified according to the centric scheme as middleware-based ITS, hardware-based ITS, and recovery-based ITS. Recent research focusing on adaptive transformation schemes is also introduced.