Okun's Law postulates an inverse relationship between movements of the unemployment rate and the real gross domestic product (GDP). In this article we investigate Okun's law for 15 OECD countries and check for its structural stability. By using data on employment and the labor force we infer whether structural instability is caused either from the the demand side or the supply side. (author's abstract) / Series: Working Papers Series "Growth and Employment in Europe: Sustainability and Competitiveness"