Peptide signaling molecules are well characterized in animal systems, but it is only over the last three decades that they have been recognized in plants. In this chapter, we compare some of the major features of animal peptide signaling molecules with the new classes that have been identified in plants. We introduce the concept of modular signaling and discuss how this adaptable feature can be evolutionarily advantageous to multicellular organisms. Most signaling peptides have been identified in angiosperms (both monocot and dicot) although representative signaling peptides occur in moss and green algae. Some classes contain peptides with highly diverse sequences (within and across species) while other peptide signaling classes are small or represented by a single peptide or only found in a single family of plants. The different classes of plant signaling peptides are not phylogenetically related indicating that they have been independently selected to enable modular or mix and match signaling.