Plants have evolved a family of unique membrane receptor kinases to orchestrate the growth and development of their cells, tissues, and organs. Receptor kinases also form the first line of defense of the plant immune system and allow plants to engage in symbiotic interactions. Here, we discuss recent advances in understanding, at the molecular level, how receptor kinases with lysin-motif or leucine-rich-repeat ectodomains have evolved to sense a broad spectrum of ligands. We summarize and compare the established receptor activation mechanisms for plant receptor kinases and dissect how ligand binding at the cell surface leads to activation of cytoplasmic signaling cascades. Our review highlights that one family of plant membrane receptors has diversified structurally to fulfill very different signaling tasks.