User interface design guidelines are not simple recipes to be applied mindlessly. Applying them effectively requires determining their applicability and precedence in specific situations. It also requires balancing the trade-offs that inevitably arise in situations when guidelines appear to contradict each other. By understanding the perceptual and cognitive psychology that underlies UX design guidelines, designers and usability evaluators enhance their ability to interpret and apply the guidelines. This presentation, based on the presenter’s book “Designing with the Mind in Mind: Simple Guide to Understanding User Interface Design Guidelines, 3rd edition”, explains that psychology, and includes many demonstrations of human perception and cognition, as well as examples of good and bad designs.