They develop and test many canine skills – obedience and control, intelligence and independence, searching and tracking, agility and fitness. Trials are physically demanding for both dog and handler, but are also great fun and extremely rewarding. Trials always take place out of doors, often in very attractive landscapes, and are run right through the year. Because Working Trials tests, especially the tracking exercises, tend to be dispersed over a relatively large area of land, trials rarely attract many spectators apart from the competitors themselves - the exception being the annual Kennel Club Working Trial Championships, the 'top' event of the Working Trials calendar. Almost any dog of any breed of  can take part in Working Trials, provided they are fit and healthy. The lower levels of competition – called ‘stakes’ – are geared to virtually any size of dog; however smaller dogs are not normally able to attain the top stakes (TD and PD) due to the height and size of the equipment used for the ‘agility’ exercises. Any Kennel Club registered dogs (pedigree or non-pedigree) can take part, but they must be 18 months old before they can compete at a Working Trial.
(taken from the KC web site)