The doctrine held in the Catholic that the Body and are together and simultaneously administered in communion even when a communicant receives one of the elements only. This doctrine is meant to justify the act of withdrawing the chalice from the laity ever since the twelfth century, and giving the Body alone, the Blood being restricted to the clergy.

This practice was introduced during the papacy of Paschal II (1099-1118), and the decree was confirmed at the twenty-first session of the Council of Trent on 16 July 1562.

The Eastern Orthodox churches, however, have always held that receiving both the Body and the Blood is essential for eucharistic communion, and accordingly reject the doctrine of concomitance.