In this chapter, section 4.1 Backpropagation and XOR, section 4.2 Using the Simulator to run BP on the XOR Example and section 4.3 Configuring the Encoder Problem provide a tutorial introduction in three parts. section 4.1 Backpropagation and XOR provides a brief overview of a connectionist learning algorithm, backpropagation, and a simple but classic learning problem that can be solved by this model, called XOR. It is worth reading this section even if you are familiar with these problems, since it introduces some of the conceptual structure behind the simulator and some important terminology, such as the names of many of the variable names used in the simulator. section 4.2 Using the Simulator to run BP on the XOR Example shows you how to use the PDP++ system to run backpropagation on the XOR example, indicating how to control the learning process, modify options and parameters, save and load results, etc. section 4.3 Configuring the Encoder Problem shows you how to to configure the backpropagation model for an example of you own. To illustrate the configuration process we choose another classic problem, the Encoder problem.