The paper records some observations on swallow holes and mines in the Chalk and their effects on road construction, drawing examples from recent motorway construction for the M4 and M40 in Berkshire and Buckinghamshire. The forms and modes of origin of swallow holes are discussed and it is shown that percolation of rainwater can dissolve enough chalk to account for the formation of the smaller examples. Besides ground survey, air photographs are effective for locating swallow holes having surface expression of some kind, but no method has so far been generally successful in detecting those that have no surface expression. The engineering significance of swallow holes is the necessity of securing carriageways and bridges against possible subsidence caused by them. Various methods that have been used are mentioned and it is suggested that in areas at risk the completed road should be kept under surveillance against the appearance of new swallow holes. The paper concludes with an account of the discovery and inspection of an old unrecorded mine in the Chalk and the engineering measures that were taken to deal with it.