Plants are nature's other successful experiment with multicellular life. To coordinate growth and development of their cells, tissues and organs plants have evolved unique plasma membrane receptor kinases (RKs). Several members of this protein family function as pattern recognition receptors, and as hormone receptors shaping the architecture of the plant. There is mounting evidence that different plant RKs are organized in membrane signaling complexes. RKs have a common structural architecture and share downstream signaling components. As such, it is presently unclear how the recognition of specific endogenous or foreign signals at the cell surface is translated into the activation of specific developmental programs or immune responses in the cytosol. We propose to combine physiology, genetics and cell biology with phosphoproteomics, quantitative biochemistry and structural biology to identify the shared and specific mechanisms by which plant developmental and immune receptor complexes are activated. We will dissect, in molecular detail, how activated receptor complexes generate specific signaling output in the cytosol and how the activity of plant RKs are regulated by inhibitor proteins. We envision that our work will provide a molecular framework for understanding how specificity is encoded at the molecular level in RK signaling, setting the stage for engineering these pathways in crops in the future.