 This entry includes a presentation of the main definitions and theorems used in the development of the logicist project in PM. The entry indicates a path through the whole work presenting the basic results proved in Principia Mathematica (PM) in a somewhat more contemporary notation, so as to make it easy to compare the system of Whitehead and Russell with that of Frege, the other most prominent advocate of logicism in the foundations of mathematics. The aim of that program, as described by Russell in the opening lines of the preface to his 1903 book The Principles of Mathematics, namely to define mathematical notions in terms of logical notions, and to derive mathematical principles, so defined, from logical principles alone: