A central element in life-cycle modeling of engineered systems is the appropriate understanding, evaluation, and modeling of degradation. In this chapter we first provide a formal definition and a conceptual framework for characterizing system degradation over time. Afterward, we discuss the importance of actual field data analysis and, in particular, we present a conceptual discussion on data collection. We also present briefly the basic concepts of regression analysis, which might be considered the first and simplest approach to constructing degradation models. Regression analysis will be used later to obtain estimates of the parameters of degradation models. As an example, the special case of estimating the parameters of the gamma process (see Chap. 5 ) is presented. This chapter is not intended as a comprehensive discussion on degradation data analysis, as this topic has been widely studied in a variety of different research fields, and many tools and procedures are available for modeling degradation data.