Spectroscopic, specular reflected light measurements (both ellipsometry-SE, and reflectometry-SR) of grating structures have relatively and recently been shown to yield very accurate information on the critical dimensions, wall-angles and detailed wall shape of deep submicron features. The technique is often called 'scatterometry' or optical critical dimension (OCD) measurement. This technique has been moved rapidly from initial demonstrations to significant industrial application. In this paper, we will review the development of this technique and ex situ applications. When applied in situ, this technique opens up exciting new opportunities for studying the evolution of topography in semiconductor fabrication processes and for applying real-time control methods for nanometer level feature size accuracy. We will briefly comment on limitations and challenges for this measurement technique. (C) 2004 Elsevier B.V. All rights reserved.