Chapters: The Physical Meaning of Poincar Massless Representations; Massless Representations; Massless Fields are Different; How to Couple Massless and Massive Matter; The Behaviour of Matter in Fields; Geometrical Reasons for the Poincar Group; Description of the Electromagnetic Field; The Equations Governing Free Gravitation; How Matter Determines Gravitational Fields; Nonlinearity and Geometry; Quantum Gravity.