A system and method are provided that incorporate subjects or subject lines into instant messaging sessions or conversations on a mobile device. During an instant messaging session, the user is provided with an option to specify a subject for at least the current portion of the session or for the entire session. The subject is proposed to the other users in the session, who can accept, ignore or modify the subject at their end. Multiple subjects can be specified for a single conversation if desired, and the subjects can be displayed with the contact in a message list. The instant messaging sessions may then be searched using search tools that are typically used for search email and other data messages.