Defining the mode of action to be modeled is the first step in QSAR generation (McKinney et al. 2000). Chapter 1 covers broadly the various modes of action. The class of relevant toxicants and candidate explanatory variables (descriptors) are identified once the mode of action is defined. Previous chapters describe the potential explanatory variables for metal ions. The final step in QSAR development is the generation of quantitative means for selecting and relating the explanatory variable(s) to the effect of interest. The purpose of this chapter is to provide essential details about this last step.