Processing of information in the cerebral cortex is characterized by cooperative interactions within distributed neuronal assemblies. Synchronization of neuronal responses with a precision of a few milliseconds is thought to organize the respective neuronal assemblies (i) by enhancing the visibility and impact of the spatiotemporal activity patterns of synchronously firing groups of neurons within the nervous system, and (ii) by enhancing and structuring the functional connectivity for the computations just needed. Often such synchronous discharges appear in oscillatory patterns within the gamma frequency range. Cognitive processes depend on the highly flexible selection and combination of information and are therefore expected to make particular extensive use of a dynamic processing architecture. Recording of neuronal activity in the cerebral cortex of macaque monkeys performing well defined cognitive tasks provide a unique opportunity to test this hypothesis. For a spatial selective attention task which requires to detect a change in speed, neurons in area MT with overlapping receptive fields showed more synchronous oscillatory firing in response to a moving bar stimulus that was a target of attention than to the same bar being a distracter. In area V4 a strong increase of synchronous oscillatory activity occurred when monkeys attended a morphing shape in the contralateral hemifield instead of a similar morphing shape in the ipsilateral hemifield. Synchronization between distant sites in the temporal cortex was observed to depend on correct performance of a memory task, indicating the importance of the synchronization mechanism in structuring the computational architecture. These results indicate that highly specific patterns of synchronization within well defined sets of neurons represent a basic mechanism for cognitive processing in the cerebral cortex.