1. The subject-matter of this paper is in some sense known; but we will try to organise, explain and reprove it, and to give examples.In essence, a space or spectrum X is "atomic" if a map / : X-*X may be proved to be an equivalence by a simple, computable test applied in one dimension; this goes back to [4] (published as [5]) and first appeared in print in [12]. That it is useful to prove X atomic and then apply the fact has been amply shown, beginning with [3].This notion is related to two others. Unique factorisation results for spaces and spectra have been considered in [6,9,14]. Here one needs the notion of an "irreducible" or "indecomposable" object X, and a slightly stronger notion of "prime".We first show that the case of "spaces" and the case of "spectra" can be considered together, by concentrating on the fact that the hom-set [X,X] is (under suitable assumptions) a profinite monoid. In this case we show that the "weaker" condition implies the "stronger", as follows.(a) If X is indecomposable then its hom-set [_X, AT] is "good", and (b) if [X,AT] is "good" then X is both "atomic" and "prime".We give some illustrative examples, including some which arise "in nature" as stable summands of classifying spaces BG. We conclude with the proofs.Related results have been obtained by M. C. Crabb and J. R. Hubbuck; we are grateful to them for letters, and also to F. R. Cohen and F. P. Peterson.2. First we unify the two cases to be considered. We will comment in Section 3.