In many instances natural swimming and flying involves organisms in sufficiently close proximity to cause significant interaction via the fluid medium. In these cases, the problem emphasized in the preceding chapters – propulsion of a body through a medium at rest – is modified to the extent that a given organism actually moves through the flow field created by its neighbors. To study such interactions, some hypothesis must be made concerning the nature of the response to the modified flow field. A passive organism will not modify movements or body geometry in response to environment, even though a modified flow field is sensed. Probably the only examples of a completely passive response are to be found among the microorganisms, and we consider two possible instances below. But the most interesting aspect of this class of problems resides in the capacity of the animal to modify its swimming or flying configuration as it senses changes in the ambient flow field, presumably in order to optimize performance. As we shall see later in the chapter, it has been suggested that such considerations can account for the arrangement of fish in schools and for the formation flying of birds. The observed stability of such groups could be a result of the active orientation of individuals relative to neighbors, but it is more natural to expect that an inherent stability comes from the optimal movements sought by each individual, leading to optimal performance of the group as a whole as members fall into place in a preferred pattern.