Hematophagous insects use air-borne chemical cues to guide them to resources such as blood-meal hosts, plants, and oviposition sites. Research that combines behavioral and electrophysiological approaches to the study of how insects find these resources can result in useful information about what chemical signals a mosquito can detect and at what airborne concentrations such compounds are effective. Such studies have helped clarify the role of lactic acid, ammonia, carbon dioxide, octenol, phenols, temperature, and humidity in the attraction of mosquitoes, tsetse flies, and ticks to blood-meal hosts. Egg raft pheromone, indoles, cresols, methyl cyclohexanol, 2-butoxy ethanol, and fatty acid esters have been examined with respect to oviposition site location and selection. Plant volatiles have received less attention but electrophysiological responses to terpenes and green plant volatiles have been observed. Information from studies of this type can be useful in the design of both attractants and more effective repellents.