The Drosophila larva is a suitable model to study olfaction due to its numerical simplicity. The peripheral olfactory system consists of just 21 pairs of olfactory sensory neurons (OSNs), each expressing one, or at most two classes of olfactory receptors (ORs) that define the receptive range of the OSN. Larvae produce robust behaviours to many odours and are easily genetically manipulated. Unlike audition, relatively little is known regarding olfaction due to its complex nature - most odours exist as multimolecular units that vary in their identity and structure, concentrations and proportions. Until recently, most olfactory research has focused on the processing of simple odours, which does not realistically model real-world odours. Presented here is one of the first in situ investigations of odour mixture processing in the Drosophila melanogaster larva. Processing of odour mixtures was explored using both electrophysiological recordings of the peripheral olfactory system and behavioural assays at the output of the system, and the effects of various factors on responses were also explored. w1118 larvae in which all OSNs were functional, and larvae with only a single class of OSN (using the Gal4-UAS expression system) were studied.At the peripheral level, mixture responses were never entirely transformed from the components, and were always as large as or greater than the response to the strongest component, and therefore the neuron was either 'seeing' just one or more than one of the components, respectively. Mixture responses across all OSNs were always additive, hypoadditive or partially suppressive, and there were no instances of synergistic or fully suppressive responses. Peripheral mixture responses were both OR- and component-dependent, as the same mixture was represented differently across OSNs.At the behavioural level, mixture responses were mostly additive, partially and fully suppressive and therefore not always predictable from the components. Interestingly, responses of larvae with only a single class of OSN were mostly predictable as there was no complexity of processing arising from the combinatorial code. When all OSNs were functional and the combinatorial code appropriately activated, mixture responses were more complex and unpredictable. Associative conditioning experiments revealed that larvae were unable to identify components from within a mixture, providing evidence that, at least at the behavioural level, mixture responses were probably synthetic, and therefore likely that interactions between the components occurred at some point along the processing pathway.Mixtures were often dominated by the component inducing the largest firing rate, and carbon chain length and vapour pressure influenced, to some degree, which component dominated. The nature of mixture responses were affected by concentration which is consistent with a model of receptor competition - at low concentrations mixture responses were additive, whilst at high concentrations responses were reduced compared to an additive response. In contrast, prior experience had little effect on peripheral responses to pure odours and mixtures, although rapid adaptation was observed during exposure. Exposure did have an effect on subsequent behavioural responses to pure odours and mixtures, providing evidence of central effects of adaptation.The data presented here provides evidence that mixture processing is extremely complex, with many factors influencing and affecting the way that the system responds to mixtures. This data reveals an unexpected complexity in a numerically simple system and with such'simple' complex odours, and indicates the likelihood of more complex responses when all OSNs are functional.