This chapter is a comprehensive tutorial on the classical Four-Step Model (FSM) travel demand model, which has been the cornerstone of transportation planning for over half a century. The FSM integrates Trip Generation (how many trips?), Trip Distribution (where do they go?), Mode Choice (how do they travel?), and Traffic Assignment (which route do they take?) into a coherent pipeline. This foundational understanding is essential for appreciating the unique advantages of the Ideal Traffic Assignment (ITA) framework. We calibrate gravity-based distribution and multinomial logit mode choice, and we link feedback to equilibrium assignment. A significant focus is placed on the theory of network equilibrium, including the formal statement of Wardrop’s principles, the seminal convex optimization formulation by Beckmann, McGuire, and Winsten (Studies in the Economics of Transportation, Yale University Press, New Haven, 1956). Central emphasis is placed on the Origin-Destination (O-D) matrix, its identifiability limits, modern inference with regularization and data fusion, and the consequences of aggregation. We conclude with algorithms for User Equilibrium, practical diagnostics, and a detailed comparison to Ideal Traffic Assignment, highlighting data parsimony, robustness, and transparency. Guidance is provided on when to use each paradigm for planning, future-demand forecasting, and Traffic Impact Assessment mini studies.