This document provides an introduction and overview of STELLA, a visually-based system modeling software. STELLA can be used to model dynamic systems in various fields including social sciences, policy issues, and environmental sciences. The document outlines how STELLA works, including its use of stocks, flows, and connectors to model system components and their interactions. Examples of models that can be created in STELLA include deforestation, climate change, and lake ecosystem models. Guidance is provided on how to define a system and its boundaries, assumptions, and causal relationships when building a STELLA model.