Publisher Summary This chapter discusses the form perception in nonhuman primates. The major methodological considerations in studying form perception are in specifying the stimulus and in insuring that the stimulus is appropriate for S. To obtain ecological validity, stimuli used should be representative of the real situations to which one wishes to generalize. This argues against presenting animal Ss with geometric forms and argues for stimuli associated with the Menzel jumping stand, the Delta Regional General Test Apparatus, and the Menzel obstruction box, which are a tree, a jungle, and a river, respectively. Nevertheless, for all their ecological validity, such situations have not provided extensive information about the mechanisms of form perception by nonhuman primates. There is no quick and easy way to determine which physical measurements have the greatest psychological relevance for pattern preference and discrimination. The most elusive contextual effect is provided by simultaneous discrimination of dissimilar forms. When S learns to make a particular response to a singly represented stimulus, it is seemingly impossible to apply this knowledge with assurance to problems in which two forms are presented, because one provides a context for the other.