Ethylene (C2H4) is a gaseous hydrocarbon molecule which is not only of great significance in various industries (such as rubber, plastics, paints, toys, and detergents), it is also the first reported gaseous plant hormone which can serve as a signal molecule. Ethylene is both biosynthesized and sensed by plants. Since it can diffuse to and affect surrounding plants, it also has the characteristic of pheromone. Responses to ethylene in plants can be either harmful or desirable depending on its concentration, stage of plant/tissue development, and the plant species. It also mediates adaptive response to a variety of stress factors such as salinity, pathogen attack, flooding, and drought. Ethylene is, however, best known for its essential role in the ripening of climacteric fruits such as tomato, apple, pear, and banana. A commercial source of ethylene, i.e., ethephon, is also used as a spray to induce flowering in pineapple plants and also to prevent lodging (bending over) in wheat plants (Box 19.1). It may be noted that ethylene is a by-product of partial combustion of organic fuels and is, therefore, present in the atmosphere as a result of forest fires, car exhaust, and volcanic eruptions. The greatest risk of working with pure ethylene is that it can explode because of its flammable nature.