ABSTRACT: Leachates are the main environmental problem originated by landfillings and the estimate of the expected production flowrates is an important factor in the design and project, both during operational period and after closure. A conceptual tool for such estimations is transient or unsteady water mass balances. The several inputs and output to and from the system are described and methods for accounting each of them are indicated. However, it is not only necessary to preview the quantity but also the quality. Thus, the biological and chemical mechanisms of leachates production are analysed. Their understanding entitles possible strategies for process control. Finally, leachates cannot anymore be discarded on the environment, percolating through the soil, for natural attenuation. For this reason, the principal methods of treatment by piping the leachates, by biological, physical or chemical methods are shortly described and analysed.