Development of the use of polymer and elastomer seals against corrosionresistant sealing surfaces as a method of joining preinsulated underground steam, hot water, and condensate systems will be discussed. Design considerations, including smoothness of sealing surfaces, ring shapes, compounds, physical properties, etc., will be reviewed. Advantages of this concept, such as making positive provisions for thermal expansion and contraction, making installation procedures simple and easy to do correctly, and reducing stresses on the system, will be outlined. Research by various component manufacturers and experience gained in installed systems during the past thirteen years will be given.