This thesis describes investigations into the phenomenon of time estimation in mechanical engineering design. Time estimating in this context refers to estimating in advance the approximate duration of a new design project, for the purpose of preparing schedules. The thesis describes background to the estimation problem, including practical and theoretical aspects of design, design management and market conditions. The research presented is based on data gathered from industrial collaborators, therefore detailed descriptions of the collaborating firms are included. A quantitative study is described which demonstrates that current estimation techniques are not infallible; and that there can be a statistically significant link between the estimated and actual completion times. A process of grounded theorising, based on expert interviews, is presented. Models of the design estimation task were generated by this process, and are included. Differences were found in the models for the estimation of times in an Engineer to Order (ETO) environment, and estimation in a Volume Manufacturing environment. The models were corroborated firstly by checking if they could be recognised and endorsed by the experts from which they were generated, and. secondly by checking if they could be recognised and endorsed by an expert not involved in the original model generation. Correspondence was found to be good. A modified model of time estimation is presented, taking into account the findings of the corroboration exercise. Finally, an example of a simple tool for assisting the estimation process is included.