A computer-implemented method comprising: detecting a current context associated with a mobile computing device (172b, 202), the context being external to the mobile computing device and indicating a current state of the device in its surrounding environment; determining, based on the current context associated with the mobile computing device indicating that the mobile computing device is located in a car, to switch the mobile computing device from a first mode of operation in which audio data is not monitored for voice input that indicates a request from a user for the mobile computing device to perform an operation, to a second mode of operation in which audio data is monitored for voice inputs; in response to determining to switch the mobile computing device to the second mode of operation, activating one or more microphones to receive ambient audio signals and provide electronic audio data to the mobile computing device, and activating a speech analysis subsystem associated with the mobile computing device to detect voice inputs from other ambient noises in the audio data and to identify whether one or more keywords from a predetermined group of keywords is present in the voice inputs to determine if any of the voice inputs is a request from the user for the mobile computing device to perform the operation; and providing output on the mobile computing device that is responsive to a voice input identified as the request from the user for the mobile computing device to perform the operation, the method further comprising: determining to switch from the second mode of operation to the first mode of operation based on the current context associated with the mobile device indicating that the user has removed the mobile computing device from the car; and in response to determining to switch to the first mode of operation, deactivating the one or more microphones and deactivating the speech analysis subsystem. detecting a current context associated with a mobile computing device (172b, 202), the context being external to the mobile computing device and indicating a current state of the device in its surrounding environment; determining, based on the current context associated with the mobile computing device indicating that the mobile computing device is located in a car, to switch the mobile computing device from a first mode of operation in which audio data is not monitored for voice input that indicates a request from a user for the mobile computing device to perform an operation, to a second mode of operation in which audio data is monitored for voice inputs; in response to determining to switch the mobile computing device to the second mode of operation, activating one or more microphones to receive ambient audio signals and provide electronic audio data to the mobile computing device, and activating a speech analysis subsystem associated with the mobile computing device to detect voice inputs from other ambient noises in the audio data and to identify whether one or more keywords from a predetermined group of keywords is present in the voice inputs to determine if any of the voice inputs is a request from the user for the mobile computing device to perform the operation; and providing output on the mobile computing device that is responsive to a voice input identified as the request from the user for the mobile computing device to perform the operation, the method further comprising: determining to switch from the second mode of operation to the first mode of operation based on the current context associated with the mobile device indicating that the user has removed the mobile computing device from the car; and in response to determining to switch to the first mode of operation, deactivating the one or more microphones and deactivating the speech analysis subsystem. determining to switch from the second mode of operation to the first mode of operation based on the current context associated with the mobile device indicating that the user has removed the mobile computing device from the car; and in response to determining to switch to the first mode of operation, deactivating the one or more microphones and deactivating the speech analysis subsystem.