Optimal performance of antennas used in multiple-input multiple-output (MIMO) systems is addressed in this communication with optimality being expressed in terms of the power radiated, which is subject to realistic, yet unknown, excitation and fixed, however, arbitrarily complicated, matching. It is shown that if excitation is not specified, the optimality of an MIMO radiating system has to be understood differently as compared with the case of a specified excitation. Consequently, two important figures of merits-transducer and radiation efficiency figures-are adopted to measure the quality of the MIMO radiating systems. The communication is accompanied by examples illustrating the theoretical concepts.