A Tsirelson bound is an upper limit to quantum mechanical correlations between distant events. Given that quantum mechanics is non-local (i.e., that quantum mechanical correlations violate Bell inequalities), a natural question to ask is "how non-local can quantum mechanics be?", or, more precisely, by how much can the Bell inequality be violated. The answer is precisely the Tsirelson bound for the particular Bell inequality in question. In general, this bound is lower than what would be possible without signalling faster than light, and much research has been dedicated to the question of why this is the case.