The paper discusses the use of flares to destroy volatile organic and toxic compounds. Flares are used to destroy industrial gases from which the heating cannot be economically recovered. Results of an EPA investigation of overall flare combustion efficiency and specific compound destruction efficiency, using a large pilot scale facility built in 1982, show that, for the gases tested, combustion and destruction efficiencies are above 98% if the ratio (lower heating value of gas being flared)/(minimum lower heating value required for stable combustion) is greater than 1.2. If this stability ratio falls below 1.2, the efficiencies fall rapidly, reaching zero when the flame goes out. The results also show that the stability limit is strongly influenced by the velocity and lower heating value of the gas leaving the flare tip and the design of the flare tip. A pilot flame can improve stability by extending the stable combustion regime to lower values of the lower heating value. Air and steam assist, used to reduce smoking, can affect combustion efficiency only if the assist gas flow rate is excessive.