Tax administrations are increasingly looking to adapt and modernise their IT systems to make them more efficient, effective and better able to provide the services that taxpayers increasingly expect. This can be a difficult undertaking as there are often large numbers of highly integrated legacy IT systems carrying out different processes which may not always work seamlessly together. Following detailed market research and after thorough consideration, the Finnish Tax Administration (Tax Finland) decided to replace its many legacy software systems with a single commercial off-the-shelf (COTS) product. The implementation of the COTS solution for Tax Finland in all its phases took approximately ten years. The experiences and the lessons learnt during the implementation process are shared by Tax Finland in this handbook as an aid to others considering similar reforms.