In this article the characteristics and requirements of a sustainable energy system are described. Special attention is given to the need to reduce greenhouse gas emissions, especially CO2. It is indicated that we may have to reduce the annual CO2 emissions due to our energy consumption from 6 GtC at present to less than 3 GtC in the year 2100, and the cumulative CO2 emissions between the years 1990 and 2100 to 550-750 GtC. To reduce the CO2 emissions, one option we can develop and apply is improvement of the energy efficiency with 50-90%. In addition strong efforts are needed to develop a sustainable energy supply system with low or no CO2 emissions. Option number one, the use of renewable energy sources, offers a huge potential but major technological breakthroughs are required to allow a massive application of these sources at affordable costs, in a reliable way and in an acceptable manner. This probably also holds for another option, nuclear energy. The impact of a fuel switch from coal to oil and natural gas will depend on the recoverable amount of fossil fuels. Therefore, due attention should be given to Carbon Dioxide Removal (CDR), especially as it is the only option that may allow continuing large scale use of fossil fuels. Application of this option could prevent the emission of 300 GtC or more between the years 1990 and 2100. Studies in the Netherlands suggest its application would be accepted socially. One of the main implementation barriers is its high cost. In the power sector CDR may increase the electricity production costs with 30-100%, although there is room to reduce these costs. Lower cost CDR options are available outside the power sector, like CDR from natural gas recovery processes, from specific industrial processes and from the production of hydrogen from natural gas, coal or biomass. Consequently, CDR may be applied at a cost ranging from US$ 5-10 to 250 per ton Carbon avoided, depending on the option and circumstances involved. Given the pursuit of sustainability, further attention is needed not only for costs reductions but also for the environmental and safety aspects of CO2 disposal especially in aquifers and in the deep ocean.