Evaluation of the explosion hazard from accidental releases in enclosures involves estimating the flammable volume produced by mixing of the released material with air. Two specific situations are considered in the paper. In the first case dealing with gaseous jet releases, recent results are discussed on the effects of simple obstacles on the flammable volume. These results illustrate how the flammable volume can increase or decrease, depending on the values of dimensionless parameters that define the release and the obstacle. The second case considered is that of the flammable layer produced by the evaporation of a liquid pool. This particular release scenario has been studied using a computational approach based on a one-dimensional model. The paper discusses the rationale behind this approximation and the adjustments that have been found to be necessary to include multi-dimensional effects into the approximate treatment. The common thread among the correlations or calculation methods discussed in the paper is the desire to identify tools that can be of use in engineering applications.