Typical MARKAL models run in 3-10 minutes depending upon size and assuming a state-of-the-art LP (Linear Programming) optimizer. MARKAL-MACRO (NLP, Non-Linear Programming) runs in 0.25-4 hours depending on model size, although advancements in computer capability tend to decrease these running times. In addition, ETSAP is currently investigating the possibility the NLP formulation into a Mixed Complementarity Programming (MCP) formulation. For a certain class of MCP problems, the use of the PATH solver for MCP leads to significantly lower solution times compared to the analogous NLP problem. The run of a base case of the (rather complex) Western European MATTER 4.2 model (including report writing) takes about 60 minutes on a computer with a 266 MHz pentium processor and the OSL optimiser. Each scenario variant with a new emission penalty takes another 20 minutes. As a reference, the Netherlands MARKAL NL97 model needs about 4 minutes to solve