The resource and power consumption of ICT is still increasing, but also the benefits of ICT, e.g. in finding more efficient solutions for environmental problems. To date, it is not clear, whether the resource and energy savings through ICT overbalance the resource and energy consumption by ICT, or not. Up to now, manifold efforts of Green IT address the environmental aspects of sustainability considering computer hardware. However, there is still a lack of models, descriptions or realizations in the area of computer software and software process models. In our contribution, we first propose definitions of the terms “Green and Sustainable Software” and “Green and Sustainable Software Engineering”, then we outline a conceptual reference model, the GREENSOFT Model. This model includes a cradle-to-grave product life cycle model for software products, sustainability metrics and criteria for software, software engineering extensions for sustainably sound software design and development, as well as appropriate guidance.