This thesis investigates how low-functioning children with autism and language-matched typically developing children comprehend symbolic relations between pictures, words and the objects they represent. Using a series of cognitive-behavioural paradigms, this research documents how these developmental populations use pictures as a source for deducing information about the world and whether they differ in their understanding of what fundamentally relates pictures and referents. Study one tests whether children's mapping of referential picture-object relations is facilitated by iconicity (the extent that a picture resembles its referent) and/or verbal labelling, and study two examines the perceptual cues that direct the generalisation of labels from colour photographs. The third study addresses the factors that influence children's ability to contextualise pictorial symbols and use them to adaptively guide their behaviour in real time and space. Study four investigates the relative importance of artists' communicative intentions and perceptual resemblance to children's mapping of word-picture and picture-object relations, and study five examines whether representational status (as determined by artists' intentions) influences children's naming and reproduction of ambiguous shapes. The findings of this thesis indicate that children with autism can recognise perceptual similarities between pictures and objects, but they do not understand the rules that constrain symbolic word-picture-object mapping. Across studies, iconicity emerges as an important factor that mediates symbolic picture comprehension in autism. Furthermore, unlike their typically developing peers, children with autism do not reflect on the social-communicative intentions underlying pictures when naming, drawing or identifying referent objects. Instead, they derive meaning based exclusively on resemblance, making them naive realists. Theoretically, this thesis contributes to the field by informing our understanding of how low-functioning children with autism, an extremely under-researched population, comprehend symbols. At an applied level, these results have important implications for the design and delivery of picture-based communication interventions.