Energy security may have different meanings. In addressing the energy security question from an economic perspective, a standard definition “as reliable and adequate supply of energy at reasonable prices” may be accepted once the meaning of “reasonable prices” is given. Since the problem concerns the society as a whole (be it a single nation or the European Community or the OECD countries), prices to be considered are those for the society as a whole. This is to say that external costs have to be added to the market prices and that they are represented by the negative impacts on climate change, i.e. GHG emissions. The Canadian tar sands are analyzed as a case study to investigate how each viable energy mix for Europe performs with respect to both energy security and climate change. The minimization of the risk of supply disruption and of GHG emissions being the optimum target. Needless to say, those choices that produce positive impacts on both are preferred ones while choices having opposite impacts need a comparison among their economic costs and benefits.