Structure is a critically important determinant of system behaviour. The relationships between the most basic elements (accumulations, physical flows, information flows, feedback and delay) found in system dynamics models and the building blocks of system dynamics structure are examined. Complexity is explained. The complexity of models we might build, relative to real-world complexity is examined. Why we need to approach model building top-down rather than bottom-up is explained. How to design system dynamics group model building projects through a top-down approach is explained. How to decompose conceptual models developed top-down into appropriate modules to be constructed then synthesised bottom-up is explained. Attendees will design a group model-building project using this approach. The relationships between model functionality, verification, the model as a necessary and sufficient representation of the real world, and the real challenges of validation are explained. How to build models and design effective test to ensure those models work as intended is explained and demonstrated. How to build models using a methodology integrating aspects of systems thinking, system dynamics modelling and engineering is demonstrated.  How to manage the complexity of the model through each stage of development is explained..