They can swing a game and occasionally have little to no effect. But since their introduction to the game, substitutions have become an important facet of football.
But are they being underused by managers?
Jonathan Liew of The Telegraph has looked into the impact of managers either not using all of their available subs or bringing them on too late.
Tonight he joined us on the show to talk about his findings, which you can read in full here.
Chelsea's Jose Mourinho used the largest percentage of substitutions over the "last season-and-a-bit" according to Liew, with Swansea manager Gary Monk coming in a close second. Meanwhile Liverpool's Brendan Rodgers was at the other end of the scale.
Liew explained how tiredness is underestimated and the true optimum times to make substitutions.
He also emphasised why all three substitutions should be used during a game rather than the situation where only one or two options are used.
Liew also spoke about the results specific to the World Cup in terms of optimal substitution times.
Listen to the full interview via the Football Show podcast which sees Miguel Delaney talking about Ireland's 37-man squad and the Qatar World Cup situation:
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