This chapter deals with the subject of estimating the costs of software maintenance. It reviews the existing literature on the subject and summarises the various approaches taken to estimate maintenance costs starting with the original COCOMO approach in 1981. It then deals with the subject of impact analysis and why it is essential to estimate the scope of maintenance projects. Examples are given to illustrate this. It then goes on to describe some of the tools the author has developed in the past ten years to support his practice of maintenance project estimation including the tools SoftCalc and MainCost. For both of these tools empirical studies of industrial experiments are presented as proof of the need to automate the estimation process.