Trojans are malicious programs that perform actions which are not authorized by the user: they delete, block, modify or copy data, and they disrupt the performance of computers or computer networks. Unlike viruses and worms, the threats that fall into this category are unable to make copies of themselves or self-replicate.

The first Trojans, which appeared in the late 1980s, masqueraded as innocent programs. Once the unsuspecting user ran the program, the Trojan would deliver its harmful payload. In the early days of PC malware, Trojans were relatively uncommon since the author had to find some way of distributing the Trojan manually.