A method comprising: displaying, by data processing hardware of a mobile computing device, a prompt in a first graphical user interface executing on the data processing hardware, the prompt requesting a user to provide a hotword for initiating the mobile computing device to perform a particular action; detecting, by the data processing hardware, an utterance from the user, the utterance comprising a hotword; displaying, by the data processing hardware, a second graphical user interface that displays a confirmation of a transcription of the utterance; generating, at the data processing hardware, a waveform that represents the detected utterance, the waveform being transmitted by the mobile computing device to a hotword configuration engine over one or more networks; obtaining, by the data processing hardware using the utterance, a hotword model that corresponds to the hotword, the hotword model being received over one or more networks from a hotword configuration engine configured to retrieve one or more pre-computed hotword models identified as corresponding to the hotword from a vocabulary database, the one or more pre-computed hotword models being identified based on the waveform that represents the detected utterance; after obtaining the hotword model that corresponds to the hotword, detecting, by the data processing hardware, the hotword in a second utterance spoken by the user using the obtained hotword model; displaying, by the data processing hardware of the mobile computing device, in response to detecting the hotword in the second utterance, a third graphical user interface, providing a description of an action to be taken associated with receiving the hotword; and initiating, by the data processing hardware, the mobile computing device to perform the particular action. displaying, by data processing hardware of a mobile computing device, a prompt in a first graphical user interface executing on the data processing hardware, the prompt requesting a user to provide a hotword for initiating the mobile computing device to perform a particular action; detecting, by the data processing hardware, an utterance from the user, the utterance comprising a hotword; displaying, by the data processing hardware, a second graphical user interface that displays a confirmation of a transcription of the utterance; generating, at the data processing hardware, a waveform that represents the detected utterance, the waveform being transmitted by the mobile computing device to a hotword configuration engine over one or more networks; obtaining, by the data processing hardware using the utterance, a hotword model that corresponds to the hotword, the hotword model being received over one or more networks from a hotword configuration engine configured to retrieve one or more pre-computed hotword models identified as corresponding to the hotword from a vocabulary database, the one or more pre-computed hotword models being identified based on the waveform that represents the detected utterance; after obtaining the hotword model that corresponds to the hotword, detecting, by the data processing hardware, the hotword in a second utterance spoken by the user using the obtained hotword model; displaying, by the data processing hardware of the mobile computing device, in response to detecting the hotword in the second utterance, a third graphical user interface, providing a description of an action to be taken associated with receiving the hotword; and initiating, by the data processing hardware, the mobile computing device to perform the particular action.