Technical Abstract: Chapter 3 is concerned with the construction of appropriate models for experimental data which can then be used in the nonlinear regression analyses. Types of experimental data and potential background contributions are discussed. The various types of equations for models # eg. closed form, not closed form, peak shapes - are presented and discussed. Finding the best model for the type of response curve obtained from an experimental instrument is extremely important in this method of analysis. An example of incorrectly analyzing spectroscopic bands with polynomial procedures instead of nonlinear regression analysis of the data using a Gaussian band shape is contrasted.