This article describes a new command, gformula, that is an implementation of the g-computation procedure It is used to estimate the causal effect of time-varying exposures on an outcome in the presence of time-varying confounders that are themselves also affected by the exposures The procedure also addresses the related problem of estimating direct and indirect effects when the causal effect of the exposures on an outcome is mediated by intermediate variables, and in particular when confounders of the mediator-outcome relationships are themselves affected by the exposures A brief overview of the theory and a description of the command and its options are given, and illustrations using two simulated examples are provided. © 2011 StataCorp LP.