About one-third of the Swiss nuclear capacity is due to be retired in the next ten years, creating a short-term supply gap. In addition, the Swiss Federal Council has decided to phase out nuclear power over the longer term by not replacing existing nuclear power plants after retirement. We have analysed possible electricity supply options for responding to these two developments under different conditions using the Swiss TIMES electricity sector model-a least-cost optimization framework. Short-term demand can be cost-effectively met with new investment in gas-fired generation capacity. However, meeting the government's CO2 emission and renewable electricity targets requires an accelerated investment in renewable generation and/or increased reliance on imported electricity. In the medium and longer term, nuclear represents the most cost-effective option. The alternatives to nuclear lead to increased dependence on imported natural gas, seasonal renewables and imported electricity. All non-nuclear supply options increase the cost of electricity supply by between 50 and 150%, and create a range of tradeoffs between supply security and climate change mitigation goal. However, it is expected that an accelerated uptake of end-use efficiency measures and demand side management would reduce future electricity demand, thus reducing the need for some expensive supply options. (C) 2012 Elsevier Ltd. All rights reserved.