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The genome of Arabidopsis thaliana encodes more than 600 receptor-like kinase (RLK) genes, by far the dominant class of receptors found in land plants. Although similar to the mammalian receptor tyrosine kinases, plant RLKs are serine/threonine kinases that represent a novel signaling innovation unique to plants and, consequently, an excellent opportunity to understand how extracellular signaling evolved and functions in plants as opposed to animals. RLKs are predicted to be major components of the signaling pathways that allow plants to respond to environmental and developmental conditions. However, breakthroughs in identifying these processes have been limited to only a handful of individual RLKs. Here, we used a Syngenta custom Arabidopsis GeneChip array to compile a detailed profile of the transcriptional activity of 604 receptor-like kinase genes after exposure to a cross-section of known signaling factors in plants,including abiotic stresses, biotic stresses, and hormones. In the 68 experiments comprising the study, we found that 582 of the 604 RLK genes displayed a two-fold or greater change in expression to at least one of 12 types of treatments, thereby providing a large body of experimental evidence for targeted functional screens of individual RLK genes. We investigated whether particular subfamilies of RLK genes are responsive to specific types of signals and found that each subfamily displayed broad ranges of expression, as opposed to being targeted towards particular signal classes. Finally, by analyzing the divergence of sequence and gene expression among the RLK subfamilies, we present evidence as to the functional basis for the expansion of the RLKs and how this expansion may have affected conservation and divergences in their function. Taken as a whole, our study represents a preliminary, working model of processes and interactions in which the members of the RLK gene family may be involved, where such information has remained elusive for so many of its members.