Critical thickness theory explains an excess yield stress in thin films inversely proportional to the film thickness. However, in other contexts, an excess yield stress is often observed which is proportional to the inverse square-root of a relevant dimension. Work-hardening coefficients show both inverse and inverse square-root behaviours. We review recent experimental work which has demonstrated these effects unambiguously, and discuss the extent to which they can be theoretically understood. (C) 2009 Elsevier B.V. All rights reserved.