In NWP, care is required in the treatment of the moisture variables in the model equations. Whether mixing ratios or specific quantities are used they need to be handled consistently in both the dynamics and the various physics parametrizations. We will describe some of the issues involved and also show how many NWP formulations omit moist forcing terms from the continuity equation. These missing terms are small but their effects may be significant where the forcing is always in the same direction, for example, in the tropical boundary layer. This could be an issue for climate modelling and seasonal forecasting.