In May 1962 the International Union of Theoretical and Applied Mechanics (IUTAM) organized a symposium in Paris on the theory of the movement of the Earth's artificial satellites designated to confront the results of the first spatial experiences with the mechanical point of view. The papers submitted during this meeting have been published in 1963 in a collection entitled "Dynamics of Satellites" by Springer­ Verlag. During the last three years the importance of studies pertaining to the dynamics of artificial satellites has continued to increase, and many results due to observations have led to a deeper knowledge of the field of forces in which these objects move, particularly the field of gravitation of the Earth, the forces due to pressure of radiation, friction of the atmosphere, etc . . . A new symposium seemed therefore suitable to determine these recent advances. However, this time it appeared appropriate to consider it more from the point of view of the interpretation obtained from the results of observations (determination of orbits and forces existing) than from the point of view of the theory of motion. For this reason the complete title of the second symposium is as follows: "Trajectories of Artificial Celestial Bodies, as Determined from Observations".