The purpose of the lecture is to provide district attorneys with an understanding of the nature of incendiary fires and the skills required in their investigations, so that they can make intelligent use of fire investigator expert witness testimony in court. The lecturer emphasizes the evidentiary characteristics of certain types of arson cases -- pyromaniac fire setting has no motive, and incendiary fires destroy their direct causal evidence. For these reasons, a successful prosecution must rely on circumstantial evidence gleaned at the fire scene and interpreted through the expertise of the fire investigator. The value of this expert witness to the prosecutor is crucial, and his testimony must be used to the best possible advantage. It is advised that proof of incendiary fire causes be given by systematically eliminating all other possible causes and then identifying the incendiary materials to imply a fire-setting act without directly stating so in the testimony. For example, the phrase 'a flammable liquid ignited by an open flame' leads to the inference of a set fire if the testimony has been structured in such a way that other explanations have been refuted. Observation of the scene is done to detect traces of the unusual and unnatural that contrast with predictable characteristics of the burning process. The lecturer describes analysis methods beginning with the location of fire origin at the area of lowest burn and proceeding from there along the path of the spreading fire. Clues are identified with the help of illustrative slides for a variety of instruments of ignition, from matches and paper to candles, tampered wiring, and Molotov cocktails. The effects of different types of fire on glass are detailed because this contains clues to the intensity of the temperature, implying a sudden explosion or a slowly smoldering fire and is tangible evidence for exhibition in court.