This book addresses physicists working in general relativity, astrophysics and cosmology. The contributions are based on reports given at a summer school the goal of which was to review modern research for students. The school was centered on the study of gravitational fields corresponding to rotating objects of astrophysical interest, under different viewpoints: theoretical, numerical and observational. Special emphasis is put on the analysis of interior and exterior fields of stationary axisymmetric systems. Lectures and contributions, collected here in Part I, ranged from basic information useful to newcomers to technical points pertaining to current research in this area. Part II contains lectures and contributions on other aspects of gravitation theory.