The summary written by The Euler Archive says: "This work is concerned with the calculus of variations. Euler's main contribution to this subject is that he changed it from a discussion of essentially special cases to a discussion of very general classes of problems. This work includes a listing of 100 special problems that Euler considers to illustrate his methods. Euler also demonstrates a general procedure for writing down the so-called Euler differential equation or first necessary condition. This is also the first work in which the principle of least action (which Euler states and discusses) is presented; the principle is the first deep insight (apart from Fermat's principle of least times) of how the calculus of variations comes into play in physics"