Eavesdropping is the intercepting and reading of messages and conversations by unintended recipients. One who participates in eavesdropping, i.e., someone who secretly listens in on the conversations of others. The origin of the term is literal, from people who would literally hide out in the eaves of houses to listen in on other people's private conversations.

Eavesdropping can also be done over phone lines, email, instant messaging, and any other method of communication considered private. (If a message is publicly broadcast, witnessing it does not count as eavesdropping.)

Messages can be protected against eavesdropping by employing a security service of confidentiality (or privacy). This security service is usually implemented by encryption.

In ancient China, it is said that to act against eavesdropping, when discussing important matters, soldiers would instead draw the characters instead, on hands or papers.

The Canadian heroine Laura Secord is famous for having eavesdropped the plans of American army and delivered this information to the British.