Microbiologically influenced corrosion (MIC) is seldom attributed to the action of just one type of microorganism, but more often to the activities of several different organisms which form a community. Because these microbial communities are common inhabitants in the environment, MIC is a problem in many industries. In the gas industry, the need is to determine whether the corrosion occurring at a particular site is MIC or not. This field guide focuses on external pipe corrosion and readily accessible internal corrosion sites. Mterials needed and sampling procedures are described. Data sheets are included for recording information and observations at a corrosion site. The guide contains photographs illustrating corrosion characteristics.