To what extent is visual attention driven by the semantics of individual objects, rather than by their visual appearance? To investigate this we continuously measured eye movements, while observers searched through displays of common objects for an aurally instructed target. On crucial trials, the target was absent, but the display contained object s that were either semantically or visually related to the target. We hypothesized that timing is crucial in the occurrence and strength of semantic influences on visual orienting, and therefore presented the target instruction either before, during, or af ter (memory - based search) picture onset. When the target instruction was presented before picture onset we found a substantial, but delayed bias in orienting towards semantically related objects as compared to visually related objects. However, this delay disappeared when the visual information was presented before the target instruction. Furthermore, the temporal dynamics of the semantic bias did not change in the absence of visual competition. These results po int to cascadic but independent influences of semantic and visual representations on attention. In addition. the results of the memory - based search studies suggest that visual and semantic biases only arise when the visual stimuli are present. Although we consistent ly found that people fixate at locat ions previously occupied by the target object (a replication of earlier findings), we did not find such biases for visually or semantically related objects. Overall, our studies show that the question whether visual orienting is driven by semantic c ontent is better rephrased as when visual orienting is driven by semantic content.