An optimal flowsheet drawing layout is one that presents a simple intuitive correspondence between the drawing and the underlying process it describes. Criteria that can be applied to make this correspondence have, however, never been clearly defined. Drawing algorithms from other disciplines are reviewed and found to have criteria quite distinct from those required for chemical process flowsheets. In most cases the criteria are not explicitly stated and heuristic solution methods are employed. The paper introduces a range of possible flowsheet drawing criteria which can be weighted to emphasize desirable drawing features. A genetic algorithm for laying out drawings is described which can easily accommodate such a wide range of optimization criteria. Results presented show that the algorithm gives "optimal" or near-optimal layouts for small to medium sized drawings (less than 20 units) which compares favourably with other genetic algorithm applications. For the first time we are able to quantify criteria for a "good" flowsheet drawing and hence clearly distinguish between the criterion and the optimization routine. One benefit of such mathematically computable criteria is that it would now be possible to develop special-purpose optimization methods applicable to larger or more complex drawings.