Connect Four is a two-player board game in which the players take turns in dropping discs into a vertical grid with the objective of getting four of one's own discs in a line. It was first published by Milton Bradley in 1974.

The game is played on a board with 7 columns and 6 rows, which is placed in a vertical position. The players have 21 discs each, distinguished by color. The players take turns in dropping discs in one of the non-full columns. The disc then occupies the lowest unoccupied square on that column. A player wins by placing four of their own discs consecutively in a line (row, column or diagonal), which ends the game. The game ends in a draw if the board is filled completely without any player winning.

Beginners will often overlook a simple threat to connect four discs; it is therefore important to always check all vertical, horizontal and diagonal lines before making a move. In more advanced play, one aims at forcing a win by making two threats simultaneously; conversely, one should prevent the opponent from doing so. For instance, in the top right figure, it is green's turn. Yellow will win on their next turn irrespective of green's move because of the threat of extending the diagonal of three consecutive yellow discs to four by dropping a disc either on the third or on the last column, only one of which can be blocked by green.

As a general rule of thumb, discs played in the center columns are more valuable than border column discs, because they participate in more potential four-disc lines (and accordingly limit the opponent's opportunities).

Among good players, the short term goal is to connect three discs, thereby preventing the opponent from playing in a certain column and creating a threat in that column. A player who manages to create two threats immediately on top of each other wins directly.

Towards the end, the game then often turns into a complex counting match: both players try to win by forcing the other to play a certain column. In these situations, it is useful to realize that, if it's your move, then after filling an even number of places, it's still your move. Every column has an even number of places.