Defining 'context' has proved to be a non-trivial problem for research in con text-awareness. In this paper we address two questions: what features of activity are required to define context? and does the use of context-awareness measurably improve user performance? The first question was addressed by a study of everyday activities, using a Photo Diary method to arrive at a set of Context Identifiers. We feel that it is important to discover what features of activity are needed in order to describe context. Two User trials were carried Out to address the second question. We Conclude that the use of context improves user task proficiency. (C) 2004 Elsevier Ltd. All rights reserved.