Flux leakage techniques were used as early as 1868 by the Institute of Naval Architects in England, where defects in magnetized cannon tubes were found with a compass ^[Dobmann] . In 1918, magnetic particle inspection was accidentally discovered ^[Hoke] ; magnetic particles (iron shavings) were held in place by a local change in magnetic flux at the surface near a defect. The development and commercialization of magnetic particle inspection followed soon after ^{[Deforest, Doane]} . One of the first suppliers of magnetic particle inspections was the Magnaflux Corporation. Magnaflux was formed in 1934 and remains a principal supplier of inspection equipment ^[Jiles].

MFL inspection systems are based on the same principles as magnetic particle inspection. The main difference between MFL and magnetic particle inspection is the use of sensors. MFL sensors, which were developed in the 1920's and 30's,(8) measure the magnetic field around a defect. Sensors allow a quantitative measurement, rather than the more qualitative information provided by particles.

The first MFL in-line inspection tool for pipelines was introduced in 1965 by Tuboscope ^{[Posakony92a]}. Since 1965, MFL tools have gained acceptance by the gas-transmission industry. As a result, MFL tools are some of the most widely used and effective tools available for in-line inspection of gas- transmission pipelines.