By definition, the concept of probability implies uncertainty. At the outset, this concept of uncertainty may appear to have no place in a field like nondestructive evaluation. Where safety and large maintenance costs hang in the balance, the need is for assurance rather than uncertainty. The presence of probability and uncertainty in engineering is, unfortunately, a matter of fact rather than speculation and it is best to deal with them in a serious manner instead of ignoring them. A discussion of probability necessitates the use of a variety of statistical concepts; some of the more fundamental concepts are described in more detail in Appendix A.