My PhD research explores a broad question: How do plants and pollinators communicate with each other and what is their communication language? I address this interdisciplinary question using general principles in animal communication, plant physiology, and information theory. Information available to pollinators can be categorized into signals and cues. Signals are traits that have evolved for the purpose of communication and provide a net fitness benefit to the communicating entities. For example, the dance and dazzling display of a peacock serves as a sexual signal to attract potential mates, which results in a fitness benefit by successfully passing on genes. Cues are generated unintentionally and can provoke a response from eavesdroppers. The carbon dioxide in human breath serves as a cue for mosquitoes to hunt their prey but this trait has no apparent fitness benefit for humans. This signal/cue communication framework is also observed in insect-plant interactions and serves as an engine for the spectacular diversity of plants and pollinators. Insect pollinators gather information using floral signals/cues to locate flowers in a noisy environment. Insects use color pigments and floral scents as long-distance attractants, but at a short distance from flowers, these same signals do not provide additional information about the concealed floral rewards. In my Ph.D. research, I discovered that floral humidity functions as a short-distance communication signal between plants and pollinators. Floral humidity guides pollinators to the nectar rewards and increases the reproductive fitness of flowers through increased pollinator visitation and outcrossing. Previous studies considered floral humidity a cue for the presence of nectar to pollinators, as a passive consequence of nectar evaporation. Using a nocturnal pollination mutualism between a hawk moth (Manduca sexta) and its preferred flower (Datura wrightii), I demonstrate that nectar evaporation and humidity are not directly linked. Instead, flowers actively produce humidity through stomata, resulting in an enormous humidity gradient in the flower. Surprisingly, the contribution of nectar evaporation to the floral humidity of Datura is minuscule. Floral humidity has positive fitness consequences for both the flower and the pollinator and therefore serves as a signal, not a cue, in this interaction. In summary, I have discovered a previously unappreciated signal between plants and pollinators, adding to the multimodal information landscape organisms utilize to communicate.