An autonomous coverage robot (11; 11') comprising: a housing; a debris monitoring system comprising: a receptacle (50; 50') defining an opening (53; 53'), the receptacle being releasably engageable with the housing to receive debris into the receptacle through the opening (53; 53'), wherein the opening (53; 53') is a substantially rectangular opening defined in a substantially vertical plane as debris is received into the receptacle (50; 50'), the long sides of the rectangular opening (53; 53') extending in a horizontal direction and the short sides of the rectangular opening extending in a vertical direction; a first emitter array (804A; 804A), supported on the housing on a first short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'); a second emitter array (804B, 804B') supported on the housing on an opposite short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'; 701; 782); a first receiver (802A; 802A') proximate to the first emitter array and arranged to receive the signal emitted by the second emitter array across the opening (53; 53') and reflections of the signal emitted by the first emitter array; a second receiver (802B; 802B') proximate to the second emitter array supported on the housing on the opposite short side of the opening (53; 53') and arranged to receive the signal emitted by the first emitter array across the opening (53; 53') and reflections of the signal emitted by the second emitter array; wherein the first and second receivers (802A, 802B'; 802A', 802B') and the first and second emitter arrays (804A, 804B; 804A', 804B') are disposed toward a top interior surface of the receptacle (50; 50'), the autonomous coverage robot being configured to monitor passage of debris into the receptacle based on received signals of the first and second receivers. a housing; a debris monitoring system comprising: a receptacle (50; 50') defining an opening (53; 53'), the receptacle being releasably engageable with the housing to receive debris into the receptacle through the opening (53; 53'), wherein the opening (53; 53') is a substantially rectangular opening defined in a substantially vertical plane as debris is received into the receptacle (50; 50'), the long sides of the rectangular opening (53; 53') extending in a horizontal direction and the short sides of the rectangular opening extending in a vertical direction; a first emitter array (804A; 804A), supported on the housing on a first short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'); a second emitter array (804B, 804B') supported on the housing on an opposite short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'; 701; 782); a first receiver (802A; 802A') proximate to the first emitter array and arranged to receive the signal emitted by the second emitter array across the opening (53; 53') and reflections of the signal emitted by the first emitter array; a second receiver (802B; 802B') proximate to the second emitter array supported on the housing on the opposite short side of the opening (53; 53') and arranged to receive the signal emitted by the first emitter array across the opening (53; 53') and reflections of the signal emitted by the second emitter array; wherein the first and second receivers (802A, 802B'; 802A', 802B') and the first and second emitter arrays (804A, 804B; 804A', 804B') are disposed toward a top interior surface of the receptacle (50; 50'), the autonomous coverage robot being configured to monitor passage of debris into the receptacle based on received signals of the first and second receivers. a receptacle (50; 50') defining an opening (53; 53'), the receptacle being releasably engageable with the housing to receive debris into the receptacle through the opening (53; 53'), wherein the opening (53; 53') is a substantially rectangular opening defined in a substantially vertical plane as debris is received into the receptacle (50; 50'), the long sides of the rectangular opening (53; 53') extending in a horizontal direction and the short sides of the rectangular opening extending in a vertical direction; a first emitter array (804A; 804A), supported on the housing on a first short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'); a second emitter array (804B, 804B') supported on the housing on an opposite short side of the opening (53; 53') and arranged to emit a signal across at least a portion of the opening (53; 53'; 701; 782); a first receiver (802A; 802A') proximate to the first emitter array and arranged to receive the signal emitted by the second emitter array across the opening (53; 53') and reflections of the signal emitted by the first emitter array; a second receiver (802B; 802B') proximate to the second emitter array supported on the housing on the opposite short side of the opening (53; 53') and arranged to receive the signal emitted by the first emitter array across the opening (53; 53') and reflections of the signal emitted by the second emitter array; wherein the first and second receivers (802A, 802B'; 802A', 802B') and the first and second emitter arrays (804A, 804B; 804A', 804B') are disposed toward a top interior surface of the receptacle (50; 50'), the autonomous coverage robot being configured to monitor passage of debris into the receptacle based on received signals of the first and second receivers.