Famous for its use of "Mockney", which sounded like Cockney slang but was made up by either George Cole or the scriptwriters. Some expressions became so well known they have since passed into the language, including "A nice little earner" (a profitable task), "'er indoors" (the wife), "give him a little slap" (beat him up).
The series was originally conceived as a vehicle for Dennis Waterman, after The Sweeney (1975) - in which Waterman co-starred - had finished the previous year. Initially, George Cole's character, Arthur Daley, was more of a secondary character, finding situations for Terry (Waterman) to get involved in each episode. But the great chemistry between the two characters quickly made itself apparent, and as a result Arthur was brought to the fore of storylines much more.
In Seasons 1 and 2, Terry McCann drove a white Mark II Ford Capri, registration SLE 71R; this car also features in the opening titles. However in some episodes he drove a white Mark I Capri (an earlier model which pre-dates the Mark II) which had the same registration number. In a publicity photograph for The Professionals (1977), taken from the book "The Complete Professionals" by Dave Rogers, George Cowley is seen standing beside a Rolls Royce... which also has the registration SLE 71R! According to DVLA records, the registration number officially relates to the Mark II Capri.