This document discusses the three systems of the brain - the primal, emotional, and rational systems - and how understanding these systems can help design more persuasive online experiences. The primal system is common to all animals and governs basic functions related to survival like food and sex. The emotional system is centered in the limbic system and processes automatic emotions and feelings. The rational system is centered in the prefrontal cortex and allows for higher-level thinking and decision making. Understanding how to engage each system can provide a psychological toolkit for online persuasion and marketing.