This book is the second volume, in a three volume set, that describes the Cassini/Huygens mission. This volume describes the in situ investigations on the Cassini orbiter: plasma spectrometer, ion and neutral mass spectrometer, energetic charged and neutral particle spectrometer, magnetometer, radio and plasma wave spectrometer and the cosmic dust analyzer. This book is of interest to all potential users of the Cassini-Huygens data, to those who wish to learn about the planned scientific return from the Cassini-Huygens mission and those curious about the processes occurring on this most fascinating planet. A third volume describes the remote sensing investigations on the orbiter.