The phenomena of fire initiation and development is described and discussed. Fire behavior in closed and vented compartments is considered along with the factors important to spread beyond the compartment. The role of furnishings and contents is reviewed and flashover is found to be principially responsible for hazardous conditions, both thermal and toxic. Exceptions to this are discussed. The use of current and emerging test methods are described. Emphasis is on the interpretation and use of engineering measurements needed in innovative design hazard assessment, in contrast to the conventional practice of performance rankings of materials by a test method.