Living systems which attain a macroscopic scale (i.e. from about one order of magnitude above to about three orders less than that of our bodies) may be expected to produce shaped objects which may in some circumstances provide very strong evidence for the existence of life. These objects are of two main kinds. One category comprises persisting parts of the living systems themselves, e.g. bones, shells, etc. There are possibly some characteristics which differentiate such products of living systems from natural objects of similar size. It is doubtful, however, whether such a criterion could ever be convincingly applied to a shape of which only a single example was known; but the argument would become much stronger if one found a number of very similar examples of a shape which was difficult to account for in terms of natural processes. The other category of life-produced shapes are those of artefacts, external to the living system. Here again, repetition of a shape would provide good evidence that it had been produced by a living being. Some human artefacts are, of course, of such a character that even a single example would be a convincing demonstration of the existence of life, e.g. a complex mechanism such as a watch.