When the predictor is a quantitative variable, we may be interested in fitting the observed data to one of the so-called regression models, which take the form of continuous “dose-response” curves. In this chapter, the simple linear regression model presented in Chap. 4 is slightly extended to cover some everyday situations in agriculture research. Specifically, examples are given regarding regression models with grouping factors, ANCOVA models, and nonlinear regression models. For nonlinear regression in R, the drc package and its facilities are briefly introduced.