1. A method comprising: for each of a plurality of activity classifications: determining activity likelihood function values for each of said plurality of activity classifications for two or more past epochs based, at least in part, on signals from one or more sensors of a mobile device; combining said activity likelihood function values to determine a likelihood function for an activity classification at a present epoch; and inferring a present activity of a user co-located with said mobile device to be one of the activity classifications based, at least in part, on said determined likelihood function for said activity classification at said present epoch.