Active Inference is a theory explaining how organisms survive by minimizing surprise through perception and action. It uses the free energy principle as a foundation, deriving formal and empirical implications for cognitive and neural processes. The text explores Active Inference's applications across various cognitive domains, linking it to Bayesian inference, predictive coding, and other frameworks. It also examines the theory's biological underpinnings, relating its computational aspects to brain structures and neurobiology and demonstrating its use in model-based data analysis to understand behaviour and neurological conditions. Finally, it compares Active Inference to existing psychology and artificial intelligence theories.