In this paper we determine the Q-gonalities of the modular curves X0(N) for all N < 145. We determine the C-gonality of many of these curves and the Q-gonalities and C-gonalities for many larger values of N. Using these results and some further work, we determine all the modular curves X0(N) of gonality 4, 5 and 6 over Q. We also find the first known instances of pentagonal curves X0(N) over C.