The purpose of this thesis is to investigate the levels of interdependency (simultaneity) between the performances of two direct opponents in professional football. More specifically, interdependency between performances is investigated at three different levels. Firstly, empirical analyses are conducted in order to assess the levels of interdependency between the performances of two teams in direct competition using team-game observations. Secondly, data on team formations and player starting positions within these formations is utilised in order to uniquely match individual players to a sole opponent on the field of play. Further empirical analyses are then conducted in order to investigate the levels of interdependency at this more isolated individual player level using player-game observations. Finally, an empirical investigation into the levels of interdependency between the performances of a positional unit (defence or attack) and their opposing team (as a collective) is conducted using positional unit-game observations. An exclusive and detailed data set ranging from the 2007-08 season to the 2010-11 season is utilised in order to estimate several production functions for teams, individual players and positional units competing in the German Bundesliga. The results in all empirical analyses confirm that the performance of the opponent is significant. At a team and positional unit level, no evidence is found to suggest that the performances of two direct opponents(s) are interdependent, however the results reveal that the recent past performances of the opponent(s) have a significant linear impact upon the performance of the subject. In particular, relative team form going in to a match is revealed to have a significant impact upon the performance of their opponent. At an individual player level, evidence is found to confirm that the performances of players in direct competition are interdependent thus supporting the sports economics theory of joint production. Specifically, the results reveal that the performances of defenders have a significant and negative impact upon the performances of their opposing attackers.