This work assesses the performance of reflector antennas under adaptive control. In this study, a conventional reflector antenna is given adaptive control by placing additional feeds in the reflector focal plane. The antenna effectively becomes a Multiple Beam Antenna (MBA). This configuration has received far less attention than the Sidelobe Canceller which achieves spatial discrimination by placing low gain elements in the periphery of the reflector. The applications that motivate this work involve rotating radar antennas subject to main beam interference or multiple targets that must be resolved within a 3-db beamwidth. The possibility of the interference being correlated with the wanted signal is also considered. Within the context of these applications the assessment addresses: The basic (main-beam and sidelobe) cancellation performance of this type of antenna. The influence of beam characteristics such as crossover levels, gain, sidelobe levels, etc. in performance. The Control Laws that avoid cancellation of a wanted signal while rejecting interference even if knowledge in signal direction is only approximate. Algorithms to solve adaptively the above Control Laws at speeds consistent with rotating antennas and rapidly changing interference. The use of the beamformer output to generate a reference signal to avoid cancellation of the wanted signal. An algorithm, so far used in the frequency domain, applied in this work to the space domain with the purpose of avoiding cancellation of the wanted signal in the presence of correlated interference such as multipath. The construction of an experimental antenna is also undertaken and it is demonstrated that adaptive nulling is readily achieved in practice.