THIS CHAPTER BUILDS UPON CHAPTERS ON THE same topic from previous editions of the ASTM STP 169 series. Due to the fundamental nature of the subject, much of what appeared before in ASTM STP 169, 169A, and 169B, authored by Lewis H. Tuthill, and 169C, authored by G. W. DePuy, appears again here. This chapter will review and update the topic as addressed by the previous authors, introduce new technology that has been developed, and include updated references.