A computer-implemented method comprising: detecting a current context associated with a mobile computing device, the context being external to the mobile device and indicating a current state of the device in its surrounding environment; identifying previous user behaviour data relevant to the detected current context, wherein identifying the previous user behaviour data relevant to the detected current context comprises identifying, from a user behaviour data repository, previous user behaviour data associated with a context similar to the current context; determining, based on at least the identified previous user behaviour data relevant to the current context, whether a user associated with the mobile computing device has at least a threshold likelihood of providing voice input to the mobile computing device; determining, based on the determination of whether the user has at least the threshold likelihood of providing voice input, whether to switch the mobile computing device from a current mode of operation to a second mode of operation during which the mobile computing device monitors ambient sounds for voice input that indicates a request to perform an operation; in response to determining whether to switch to the second mode of operation, activating one or more microphones and a speech analysis subsystem associated with the mobile computing device so that the mobile computing device receives a stream of audio data; and providing output on the mobile computing device that is responsive to voice input that is detected in the stream of audio data and that indicates a request to perform an operation. detecting a current context associated with a mobile computing device, the context being external to the mobile device and indicating a current state of the device in its surrounding environment; identifying previous user behaviour data relevant to the detected current context, wherein identifying the previous user behaviour data relevant to the detected current context comprises identifying, from a user behaviour data repository, previous user behaviour data associated with a context similar to the current context; determining, based on at least the identified previous user behaviour data relevant to the current context, whether a user associated with the mobile computing device has at least a threshold likelihood of providing voice input to the mobile computing device; determining, based on the determination of whether the user has at least the threshold likelihood of providing voice input, whether to switch the mobile computing device from a current mode of operation to a second mode of operation during which the mobile computing device monitors ambient sounds for voice input that indicates a request to perform an operation; in response to determining whether to switch to the second mode of operation, activating one or more microphones and a speech analysis subsystem associated with the mobile computing device so that the mobile computing device receives a stream of audio data; and providing output on the mobile computing device that is responsive to voice input that is detected in the stream of audio data and that indicates a request to perform an operation.