Let us put the monitoring and control requirements into a broad perspective. If we put down a QWOT stack of layers centered in the visible spectrum, it would require about 13,000 layers to make one millimeter of physical thickness! If we were trying to control each layer to about 1% of its nominal thickness, that would be about 1 millionth of a millimeter or one nanometer. The current thrust for “very small” detail in the WIDTH of semiconductor structures is now less than one micrometer. Therefore, we are working in the realm of two or three orders of magnitude smaller than “very small.” The monitoring and control of what we are depositing to this type of tolerance is the subject of this chapter.