This chapter describes the base model used throughout this book. The basic concept of building the model and options for the purpose of the model are shown. First, we discuss the validation of price formation and the verification of agent modeling. Next, we discuss the suitable complexity level of the model depending on its purpose. The aim of these models is not to forecast accurately but rather to design a financial market that works well. The simplicity of the model is very important because unnecessary replication of macro phenomena leads to models that are overfitted and too complex. After that, we present the details of modeling agents and a financial exchange. We discuss which investment strategies agents should adopt, how agents determine the number of orders, whether agents should be implemented with learning process, and how to determine the parameters. Chiarella and Iori’s model (Chiarella and Iori, Quant Financ 2(5):346–353, 2002) is very rare in that, although it adopts an actual model to determine market prices, it is a very simple agent model. However, it is not able to treat large fluctuations of prices such as turmoil. Therefore, we build a new base model inspired by Chiarella and Iori’s model that overcomes these limitations.