Overcrowding or crowding is the condition where more people are located within a given space than is considered tolerable from a safety and health perspective. Safety and health perspectives depend on current environments and on local cultural norms. Overcrowding may arise temporarily or regularly, in the home, in public spaces or on public transport. Overcrowding in the home can cause particular concern, since the home is an individual's place of shelter.

Public transport is a system of transport for passengers by group travel systems available for use by the general public unlike private transport, typically managed on a schedule, operated on established routes, and that may charge a posted fee for each trip. There is no rigid definition of which kinds of transport are included, and air travel is often not thought of when discussing public transport—dictionaries use wording like "buses, trains, etc." Examples of public transport include city buses, trolleybuses, trams and passenger trains, rapid transit and ferries. Public transport between cities is dominated by airlines, coaches, and intercity rail. High-speed rail networks are being developed in many parts of the world.