GLn (R) actions on Posn(R) -- Measures, Integration, and Quadratic Model -- Special Functions on Posn(R) -- Invariant Differential Operators on Posn(R) -- Poisson duality and zeta functions -- Eisenstein Series: First Part -- Geometric and Analytic Estimates -- Eisenstein Series: Second Part.Posn(R) and Eisenstein Series provides an introduction, requiring minimal prerequisites, to the analysis on symmetric spaces of positive definite real matrices as well as quotients of this space by the unimodular group of integral matrices. The approach is presented in very classical terms and includes material on special functions, notably gamma and Bessel functions, and focuses on certain mathematical aspects of Eisenstein series.