Food chain length is a central characteristic of ecological communities that influences community structure, primary production, nutrient cycling, and the bioaccumulation of contaminants to top predators. Ecologists have worked for over 80 years to explain natural variation in food chain length but, until recently, progress was very slow. Recent studies have capitalized on natural abundance stable isotope techniques to make rapid progress in testing a number of the long-standing hypotheses for variation in food chain length. Here I briefly outline the history of the application of stable isotope techniques in food web ecology and the challenges of estimating food chain length. Then I discuss how stable isotope techniques have allowed us to test the influence of resource availability, ecosystem size, and disturbance on food chain length in lakes, streams and on islands. Finally, I discuss future use of stable isotope techniques in food web ecology and future tests of food-chain length theory.