Over the past decade, the number of experimental papers reporting physiological plasticity in primary neocortical regions following certain types of controlled sensory experience, have increased greatly. These reports have been characterized by specific changes in receptive fields of individual neurons and/or the distributions of receptive fields across cortical maps. There is a widespread belief that these types of plasticities have underlying Hebbian/covariance induction mechanisms. This belief appears to be based mainly on: (a) indirect evidence, largely from experiments on the kitten visual cortex, indicating that Hebbian induction mechanisms could be involved in neocortical plasticity; (b) the observation that some types of plasticity in systems other than neocortex follow Hebbian rules of induction; and (c) the adaptability of Hebbian induction mechanisms to models of neural plasticity. In addition, some experiments have directly tested the role of Hebbian induction mechanisms in experience-dependent neocortical plasticity. The present review critically analyzes these (and related) experiments, in order to evaluate the evidence for the Hebbian Hypothesis in experience-dependent physiological plasticity of neocortex. First, we present a set of criteria to show the involvement of a Hebbian process in any form of plasticity. Next, we compare evidence from each primary neocortical region to these criteria. Finally, we examine unresolved issues. While selected developmental studies are included, emphasis is placed on plasticity in the adult neocortex. It is concluded that there is some evidence meeting the criteria for the Hebbian hypothesis in neocortical plasticity. However, this evidence is quite limited considering the popular belief in the validity of the Hebbian hypothesis.