Scotland Yard is a board game in which a team of players, as "police", cooperate to track down a player controlling a "criminal" around a board representing the streets of London. It is named after Scotland Yard, the headquarters of London's Metropolitan Police Service.

Taxi, bus and underground 'tickets'

Gameplay

London (plan of the Scotland Yard board game)

One player controls "Mr. X", a criminal whose location is only revealed periodically, and the other players each control a detective, which is always present on the board.

All players start with a number of tokens allowing them a certain number of moves using the following methods:

Taxis are expensive and only allow the player to move a short distance per unit of 'currency' [card/token]. However, taxis can be used to reach any point in London, most of which is not accessible in this game by bus or tube. 
Buses are available throughout most of the map, allowing longer-distance travel more quickly if the player is located at a bus-stop. 
The London Underground in this game also allows quick travel between distant points of London. 
Water routes are available, which only Mr. X can use, using the water buses' routes along the Thames between Greenwich and Whitehall. When Mr. X uses a water-borne transport, the number of locations he can be at is narrowed quite considerably. Therefore, when Mr. X uses a "valid on any transport" black card, it is often the subject of debate whether this was used to obscure a river trip. 
Once each transport token is used by a detective, it is turned over to Mr. X, effectively giving him unlimited transport. Mr. X also has a small number of 'valid on any transport' tickets, and 'move twice this turn' tickets.

After every four moves, Mr. X has to reveal his current position. Detectives will take this opportunity to refine their estimates for his position, and plan ways to encircle him if possible. From each known position, the types of transport used by Mr. X limit the number of possible locations he may be standing in, which provides useful information to detectives (as well as preventing some types of cheating by the fugitive player).

The game is won by the detectives if they catch their man by landing on the same square as Mr. X's current location, or it may be won by Mr. X if he remains out of the grasp of detectives for the duration of the game (24 moves).


General
Scotland Yard is interesting due to the fact that the game is asymmetrical. In most board games, the players have the same goal, but in Scotland Yard the goals of the detectives and Mr. X are quite different.

One common theory is that with 3 or fewer detectives, it is relatively easy for Mr. X to win the game. With 4 or more detectives, the chances of X winning are remote. This disparity and ease of prediction makes the Scotland Yard less popular amongst 'expert' adult game-players.


Implementations
Ravensburger is the official publisher of the game, and it is commonly available in UK and US toystores and bookshops 
There is a GPL-licensed computer game called London Law based on Scotland Yard. 
Another open source implementation of the game, in Java, is available at Sourceforge. More details on the implementation of this game can be found on this page. 
Live Action Scotland Yard is a live-action game based on Scotland Yard, taking place in Toronto once every two weeks in the summer.