Double Beta Decay

We review recent developments in double-beta decay, focusing on what can be learned about the three light neutrinos in future experiments. We examine the effects of uncertainties in already measured neutrino parameters and in calculated nuclear matrix elements on the interpretation of upcoming double-beta decay measurements. We then review a number of proposed experiments.Comment: Some typos corrected, references corrected and added. A less blurry version of figure 3 is available from authors. 41 pages, 5 figures, submitted to J. Phys.