The basic governing equations of geophysical fluid dynamics have been known for many years, but how meaningful these equations are to our understanding of coastal and open oceans depends crucially on the accuracy of constitutive equations that relate the internal and external forces acting on these bodies of water. Field measurements, whether Eulerian or Lagrangian, have played and continue to play a central role in two important aspects of geophysical fluid dynamics. First, the data collected often point to the presence of various stable and coherent structures, eddies, or streams, whose study naturally becomes the focus of analytical and computational efforts. Second, the availability of good data serves to calibrate how the response of bodies of water to forces should be modeled. This book provides a substantial collection of well- written articles on how Lagrangian data are collected, analyzed, and eventually assimilated into models.