The present invention relates to voice-activated vehicle control, and to the control of UAVs (unmanned air vehicles) using speech in particular. A method of controlling a vehicle is provided that includes receiving one or more instructions issued as speech and analyzing the speech using speech recognition software to provide a sequence of words and a word confidence measure for each word so recognized. The sequence of words is analyzed to identify a semantic concept corresponding to an instruction based on the analysis, and a semantic confidence level for the semantic concept identified derived at least in part with reference to the word confidence measures of the words associated with the semantic concept. A spoken confirmation of the semantic concept so identified based on the semantic confidence level is provided, and the semantic concept is used to provide a control input for the vehicle.