The most common of the diffuse explosions are created when natural gas escapes in quantity into a confined space and mixes with the air in the space. Other diffuse explosions result from malfunctioning steam and furnace mechanisms, compressed air or gases, synthetic gases, chemicals, and dust from such materials as plastics, metals, grain, coal, and sugar. Vapor explosions are commonly grouped in two categories: those resulting from lighter than air gases and those caused by gases heavier than air. Concentrated explosions are those which occur at a precise point through such items as a firecracker, dynamite, or a small container. The clues to these explosions are the containers themselves. Common explosives are divided into the categories of (1) class A, which have highly sensitive detonation characteristics; (2) class B, which are characterized by rapid combustion rather than detonation; (3) class C, which are manufactured articles containing class A, class B, or their combination in restricted quantities; (4) primary high explosives; (5) secondary high explosives; and (6) low explosives or propellants. Militant explosives are those most often used by terrorists, notably the plastics explosive, which is stable and may be molded into various shapes. The clues to look for in an explosion are any physical evidence of forces exerted on structural components of the building or its contents. Eleven bibliographic entries are provided.