This document discusses systems and their components. A system is defined as an organized collection of interdependent parts that work together to perform a function. Systems can be open, closed, or isolated depending on whether matter and/or energy are exchanged across their boundaries. All systems have inputs, outputs, internal flows and transformations of energy and matter, and storage areas. Understanding systems through a holistic approach can provide deeper insights than examining their parts individually. Models are used to represent real systems, and can be evaluated based on their ability to explain past observations, agree with other models, and accurately predict future behavior.