As the subtitle of this study emphasizes, it is a reconstruction of Leonhard Euler’s principle of angular momentum (in its general formulation called PAM) from its early stage (as PAMfix, governing rotation around a space-fixed axis) to its growth into an established concept (as a principle of rotation around a variable axis, called PAMvar). The study relies on hitherto unpublished notebooks and manuscripts as well as his publications and correspondence. It is the first comprehensive historiographic investigation of this important concept of physics across all of Euler’s immense work. Below is a synoptic overview of how I carried out this study and the main results it has yielded.