Judo is performed on shock-absorbing mats of compressed rubber, covered in a tough non-slip material.   The mats are usually an olive green colour, but nowadays any colour is possible.

A judo contest takes place within a matted area between 8m and 10m square, surrounded by a 1m wide "danger area", denoted by red mats.   Together these mats make up the "contest area".   Surrounding the contest area is a further margin of mats up to 3m wide which is the "safety area".

All action must take place within the contest area.   If either player steps out of the contest area into the safety area, the contest is stopped and the players are brought back to the centre of the mat.   However, deliberately stepping into the safety area is not allowed and incurs a penalty.