American psychologist and psychiatrist David Ausubel is well known for his famous quote: "The most important single factor influencing learning is what the learner already knows. Ascertain this and teach him accordingly" (Ausubel, 1968, vi). But few know about the richness and importance of his assimilation theory of meaningful learning and retention, which holds many more instruction-altering insights. One of the main reasons why this theory is so important is because it focuses on the end goal teachers are after: teachers don't want students to memorise distinct ideas; teachers want students to develop vast bodies of knowledge in the subjects they are taught. Ausubel explains that the only way to achieve this is through supporting students to learn meaningfully. This book explores the key elements of the theory and what it means to learn meaningfully. It then links the theory to highly practical implications teachers can use day-to-day in all aspects of their teaching.