A fly-in, also called an aviation meet, is a pre-arranged gathering of aircraft, pilots and passengers for recreational and social purposes.

Fly-ins may be formally or informally organised, members of the public may or may not be invited, the gathering may be at an airport or in a farmer's field.

Fly-ins can be aimed at specific aircraft classes, such as taildraggers, warbirds, experimental aircraft or specific aircraft models. They may be organized by a national organization, such as the Canadian Owners and Pilots Association or the Experimental Aircraft Association, the airport owner or authority, a flying club, an aircraft type club or by a group of friends meeting perhaps for a barbecue and to socialize.

The term "fly-in" is not formally defined in the aviation legislation of many countries and it may refer to a range of events, while in others it has a specific legal meaning. For example, in Canada fly-ins may not legally include air displays or competitive flying.