Sensory systems are organized hierarchically. Early stages format transduced signals and successive processing steps perform complex computations to extract relevant sensory representations. This feedforward hierarchy is broken by cortical projections that terminate in early processing areas. In the olfactory system the descending inputs from the cortex vastly outnumber the afferent received from the sensory periphery. Therefore, revealing how these projections contribute to the coding of complex stimuli including odorant concentrations and their mixtures is necessary to understand sensory processing throughout the olfactory system. We expressed the calcium indicator GCaMP6f in the anterior piriform cortex and used multiphoton imaging to measure the stimulus-response properties of cortical projections to the olfactory bulb (OB) in awake mice. We used two sets of odorant stimuli that revealed surprising aspects of how odorants, and their mixtures, are represented in cortical projections to the OB. First, monomolecular odorants spanning a concentration range of four orders of magnitude evoked responses in feedback projections that, as a population, reflected concentration invariance. However, at the level of individual boutons, we observed responses that had complex and non-monotonic concentration dependence that favored a select concentration range. We next imaged bouton responses to odorant mixtures that contained between 2 and 12 components. When presented with complex mixtures of odorants, the activity of cortical projections rarely exceeded the activity evoked by individual mixture components and was representationally distinct from component odorants. As a reference, we imaged the same panel of odorant mixtures in the olfactory epithelium, where we found a strong relationship between representational similarity and mixture complexity. Our current studies reveal how behaviorally relevant mixture information is inherited by the OB from the cortex.