This app is a fantastic distillation of the essential concepts of calculus, including a brilliant design that links to other supporting concepts explained within the program. Buttons to expand explanations when needed keep from overwhelming the student and allow deeper investigation when desired. Supporting problems explore the various dimensions of each concept, insuring that the learner can see the skill used in a variety of core examples. True, it does not cover every last concept. But this is, in fact, a good thing. By focusing on fundamental calculation techniques, as well as the reasoning behind them and why they matter, learners can focus on essentials. I have been using this app as an aid through both calc I and calc II. So pleased I found this app!!