Mobile handheld and wearable computer systems to support fieldworkers (e.g. aircraft mechanics) doing a variety of tasks have been designed and studied by Carnegie Mellon University (CMU) researchers since 1991. We developed an initial research agenda and found some important results associated with the design and use of such systems. This article summarises some of the key findings from CMU's research and describes the results from one heuritic usability study about mobile handheld and wearable computer systems. Based on these research results, we put forward a new research agenda addressing important issues still to be resolved.