Within the last years development of greenhouse gas reduction strategies has become a vital part of both international and national planning. When developing energy and environmental strategies it is highly relevant to gain an insight into how to prioritise among various technology options available. One of the tools available for this purpose is the construction of cost-reduction curves, relating the marginal cost of CO2 reduction and the quantity of the reduced emission to specific technology options. This paper outlines different approaches for establishing cost-reduction curves for CO2 emissions, and a case based on the Danish Energy21-plan is analysed illustrating one of these approaches. The analysis is carried out from the broad perspective of the overall energy system, but with the main emphasis on renewable energy technologies, especially the use of wind energy and biomass in the Danish system; The paper concludes that the construction of reduction cost curves is helpful in two ways: (1) to decompose existing energy strategies (scenarios) into individual reduction options and (2) to compare new options with an existing strategy. In both cases the reduction cost curve highlights those technological options that have the highest reduction potential and the lowest marginal costs, and vice versa. Based on the case of the Danish energy plan, the results of the analyses show that a number of reduction options with significant reduction potentials are available at relatively low marginal costs. Among others can be mentioned increased use of combined heat and power (CHP), substituting conventional coal-fired CHP plants with biomass ones and the development of offshore wind turbines: