The "classical" parking functions, counted by the Cayley number (n + 1) n−1 , carry a natural permutation representation of the symmetric group Sn in which the number of orbits is the Catalan number 1 n+1 2n n . In this paper, we will generalize this setup to "rational" parking functions indexed by a pair (a, b) of coprime positive integers. We show that these parking functions, which are counted by b a−1 , carry a permutation representation of Sa in which the number of orbits is the "rational" Catalan number 1 a+b a+b a . We compute the Frobenius characteristic of the Sa-module of (a, b)-parking functions. Next we study q-analogues of the rational Catalan numbers, proposing a combinatorial formula for 1[a+b]q a+b a q and relating this formula to a new combinatorial model for q-binomial coefficients. Finally, we discuss q, t-analogues of rational Catalan numbers and parking functions (generalizing the shuffle conjecture for the classical case) and present several conjectures.