In-line inspection equipment is commonly used to examine a large portion of the long distance transmission pipeline system that transports natural gas from well gathering points to local distribution companies. A piece of equipment that is inserted into a pipeline and driven by product flow is called a ''pig''. Using this term as a base, a set of terms has evolved. Pigs that are equipped with sensors and data recording devices are called ''intelligent pigs''. Pipelines that cannot be inspected using intelligent pigs are deemed ''unpigable''. But many factors affect the passage of a pig through a pipeline, or the ''pigability''. The pigability pipeline extend well beyond the basic need for a long round hole with a means to enter and exit. An accurate assessment of pigability includes consideration of pipeline length, attributes, pressure, flow rate, deformation, cleanliness, and other factors as well as the availability of inspection technology. All factors must be considered when assessing the appropriateness of ILI to assess specific pipeline threats.