In this paper, a characterization of Lyapunov graphs associated to smo
oth flows on surfaces is presented. We first obtain necessary and suff
icient conditions for a Lyapunov graph to be associated to Morse-Smale
flows and then generalize them to smooth flows. The methods employed
in the proofs are of interest in their own right for they introduce th
e use of the Conley index in this context. Moreover, an algorithmic ge
ometric construction of flows on surfaces is described.