Differential geodesy is concerned with the geometry of the gravity field of the Earth, which is of fundamental importance to both theoretical geodesy and geophysics. This monograph presents a unified treatment of the foundations of differential geodesy as proposed originally by Antonio Marussi and Martin Hotine in their work. The principal features of the Marussi-Hotine approach to theoretical aspects are given in the first five chapters (based on leg calculus), while the last five chapters are devoted to the fundamental ideas of the Marussi and Hotine theory. The text includes practical problems and is intended for use by research geodesists, graduate students in geodesy, and theoretical geophysicists.Differential geodesy is concerned with the geometry of the gravity field of the Earth, which is of fundamental importance to both theoretical geodesy and geophysics. This monograph presents a unified treatment of the foundations of differential geodesy as proposed originally by Antonio Marussi and Martin Hotine in their work. The principal features of the Marussi-Hotine approach to theoretical aspects are given in the first five chapters (based on leg calculus), while the last five chapters are devoted to the fundamental ideas of the Marussi and Hotine theory. The text includes practical problems and is intended for use by research geodesists, graduate students in geodesy, and theoretical geophysicists.I. Role of Coordinates in Geodesy and Geometry.- 1. Coordinates in Geodesy and Geometry.- 2. Marussi-Hotine Approach to Geodesy.- 3. Classical Formulations of the Tensor Calculus.- 4. Abstract Notion of a Tensor.- 5. Preview of the Leg Calculus 39 Problems for Chapter I.- II. Ricci Calculus.- 1. Introduction.- 2. 3-Leg Algebra.- 3. Congruences.- 4. Geometry of Congruences.- 5. Ricci Coefficients.- 6. Calculation of Ricci Coefficients.- 7. Commutativity of Leg Derivatives.- 8. Lie Bracket.- 9. Jacobi and Schouten Identities.- 10. Interpretation of Riclñ