The game is ever-changing, and the role of the fullback is becoming increasingly demanding.
Seldom is a number three able to sit back and defend for 90 minutes, with some now spending as much time in the oppositions half as their own.
Everyone loves to see attacking, free-flowing football nowadays, and the role of the wing back or full back is an integral part of allowing that to happen.
You can read RealSport’s best number fours feature HERE.
Defence to attack and back again... it's part-and-parcel of the role, and over the course of a game, full backs in some sides may well cover more or just as much ground as a central midfielder.
Here are five players who can make a massive difference at both ends of the pitch.