The history of engineering is unusual among historical disciplines in that while it is, like all its relatives, ultimately dependent for its sources of information on documents, pictures, and artefacts, it is by no means restricted to them. Resort can be made, when appropriate, to what is referred to in this essay as retrospective analysis, not a term I like very much but usefully descriptive all the same. The laws of mechanics being eternal, it is sometimes feasible to analyse engineering history by the equations and techniques worked out for modern problems. In the main, of course, one is dealing with challenges from engineering's early history since for relatively recent times conventional sources are the adequate and proper ones to use; yet even then there are exceptions.Broadly speaking there are four types of retrospective analysis. The choice of which to deploy will depend on a variety of factors to do with the type of engineering involved and the nature of the problem it is proposed to investigate. Sometimes different retrospective techniques can be applied in combination. The categories can be summarised as follows: