We wrote this book to help you understand what is happening and why it happens before, during, and after a dive. Many of you will be preparing for an exam to become a dive professional. This is book tells you all you need to know, no more, no less. We will start off easy with a short introduction to the dive environment, where we look at tides, currents, waves, coasts, ecosystems. Why are there usually two tides per day, but only one Moon? Why do currents follow a certain pattern over the globe? What makes waves big, how do they break at the beach? How many different types of coasts are there, and why? How do marine biologists talk about the marine life they study and describe? Next, we go on with the physics of diving. We will keep the numbers to a minimum, and we promise: no formulas. We will show you how to use your experience as a diver and your common sense to understand and calculate everything. If you have a fear of physics and calculations, as we know many of you have, we will cure you from it. Give it a go. You will calculate buoyancy, air consumption, pressure, and partial pressure with a smile on your face. Well, perhaps that is too much to ask. Without sweating, let's settle for that. Next, we have a look at equipment, but because manufactures can give you so much more information than we can, and because we know you love shopping or looking at brochures, we keep it to the minimum. We tell you about tanks and tank maintenance, burst disks, balanced and unbalanced regulators, venture valves, pilot valves, up-stream and down-stream valves, and types of depth gauges. After this, we are ready to understand what happens in your body when you go diving. In the physiology of diving, we will have a look at blood, hearts, lungs, ears, and all the things that can go wrong. More importantly, we will give you the knowledge you need to respond when things go wrong, and even more importantly, how to avoid things going wrong. That does not mean you won't need an Emergency First Responder course. You do, because you need skills and practice. But you will know all you need to know. Finally, we can bring it together and talk about decompression theory, how tables and dive computers work. You will know how compartments, half times, M-values are used to make models for your tables or computers to keep you safe. We did even more. We made an on-line course with videos and many more exercises to help you study. This is also the place where people all over the world taking this course help each other with questions and answers. Visit the on-line course at www.udemy.com/easydivetheory/. You can visit the Facebook page of the book and the course at https: //www.facebook.com/easydivetheory?fref=t