We prove old and new results on the complexity of computing the dimension of algebraic varieties. In particular, we show that this problem is NP-complete in the Blum-Shub-Smale model of computation over C, that it admits a sO(1)DO(n) deterministic algorithm, and that for systems with integer coe cients it is in the Arthur-Merlin class under the Generalized Riemann Hypothesis. The rst two results are based on a general derandomization argument.