Given a multivariate polynomial P (X 1 , . . . , X n ) over a finite field F q , let N(P ) denote the number of roots over F n q . The modular root counting problem is given a modulus r, to determine N r (P ) = N(P ) mod r. We study the complexity of computing N r (P ), when the polynomial is given as a sum of monomials. We give an efficient algorithm to compute N r (P ) when the modulus r is a power of the characteristic of the field. We show that for all other moduli, the problem of computing N r (P ) is NP-hard. We present some hardness results which imply that our algorithm is essentially optimal for prime fields. We show an equivalence between maximum-likelihood decoding for Reed-Solomon codes and a root-finding problem for symmetric polynomials.