We prove a long-standing conjecture on random-cluster models, namely that the critical point for such models with parameter $qgeq1$ on the square lattice is equal to the self-dual point $p_{sd}(q) = sqrt q /(1+sqrt q)$. This gives a proof that the critical temperature of the $q$-state Potts model is equal to $log (1+sqrt q)$ for all $qgeq 2$. We further prove that the transition is sharp, meaning that there is exponential decay of correlations in the sub-critical phase. The techniques of this paper are rigorous and valid for all $qgeq 1$, in contrast to earlier methods valid only for certain given $q$. The proof extends to the triangular and the hexagonal lattices as well.