This document discusses green computing and virtualization maturity models. It begins by defining four levels of virtualization maturity - from level 0 with no virtualization up to level 3 which involves cloud computing. Each subsequent level provides increased opportunities for energy efficiency and cost savings by better utilizing computing resources. The document then examines each level in more detail, outlining principles and technologies used at each level to reduce energy usage and environmental impact. Finally, it discusses considerations for improving efficiency within cloud computing platforms and applications.