The intrinsic nature of the relationship between herbivorous insects and host plants is chemical. Plant odors, usually called chemical information, mediate the process by which herbivorous insects search for hosts. Based on the introduction of plant odors and the diffusion model, this paper summarized the olfactory orientation function of plant odors in host-seeking behavior of aboveground adults and larvae as well as belowground herbivorous insects, and we established some problems that should be given attention in the future research about the relationship between herbivorous insects and host plants. From the point of view of the relationship between plants and environmental factors, the plant odors contain constitutive and inducible odors, and we establish their correlations and differences based on the relevant concepts. The components of constitutive odors vary with plant taxonomic status. The components of inducible odors vary with the stress of herbivore infestation, phytopathogenic microorganisms, and mechanical injury. These changes differ with the plant genus and species, plant developmental stage, and the nature and mode of action of the stress factors. All plant odors showed rhythm in their release. The process of odor diffusion is complex, and the state of diffusion can be characterized by mathematical models. Plant odors have a guiding specificity for the host-seeking behavior of adults of aboveground phytophagous insect. This paper focuses on two hypotheses for the development of this specificity. Lepidopteran larvae can use phytochemical information materials to approach host plants or avoid non-host plants. The search of belowground herbivorous insects for hosts is not only related to the secondary metabolites released or secreted by the underground tissues of host plants, but also to some primary metabolites. CO₂ in the primary metabolites plays the role of "search trigger". This work may contribute to a better understanding of the chemical relationship between insects and plants and also provide references for regulating host selection behavior in herbivorous insects which seek hosts through plant odor.