Although it is now generally accepted that the thalamus is more than a simple relay of sensory signals to the cortex, we are just beginning to gain an understanding of how corticothalamic feedback influences sensory processing. Results from an increasing number of studies across sensory systems and different species reveal effects of feedback both on the receptive fields of thalamic neurons and on the transmission of sensory information between the thalamus and cortex. Importantly, these studies demonstrate that the cortico-thalamic projection cannot be viewed in isolation, but must be considered as an integral part of a thalamo-corticothalamic circuit which intimately interconnects the thalamus and cortex for sensory processing.