This paper considers fatigue cracks after the stage of initiation and refers mainly to conditions under which continuous propagation does not occur. Examples of non-propagating cracks in notched round bars, plates and thin sheets are given; the role of such cracks in accounting for much of the difference between theoretical and experimental fatigue stress-concentration values is emphasized. The discontinuous nature of growth of slowly growing cracks is shown. The maximum length of non-propagating cracks varies with the material; mild steel gives longer ones than aluminium alloy L 65, whereas so far they have not been found at all in copper (although very slow rates of growth are recorded). Attention is drawn to the way in which strain-ageing can influence the course of a fatigue crack. A satisfactory explanation of fatigue-crack growth must also account for the behaviour of non-propagating cracks.