Ca plays an important role in nodule formation in legumes. Nitrogen is an essential element required in plants and many legumes, unable to fix nitrogen independently, pair symbiotically with nitrogen-fixing bacteria that reduce nitrogen to ammonia. This legume-Rhizobium interaction establishment requires the Nod factor that is produced by the Rhizobium bacteria. The Nod factor is recognized by the root hair cells that are involved in the nodule formation in legumes. Ca responses of varied nature are characterized to be involved in the Nod factor recognition. There is a Ca flux at the tip of the root hair initially followed by repetitive oscillation of Ca in the cytosol and also Ca spike occurs around the nucleus. DMI3, an essential gene for Nod factor signaling functions downstream of the Ca spiking signature, might be recognizing the Ca signature. Further, several CaM and CML genes in Medicago and Lotus are expressed in nodules.