This paper discusses the results from the twelfth meeting of the Energy Modeling Forum (EMF) with regards to MARKAL, a linear programming model that optimizes a network representation of an energy system with respect to specified goals. It was developed in an international effort involving representatives of fifteen countries as part of the Energy Technology Systems Analysis Program (ETSAP) of the International Energy Agency. Originally written as a main-frame computer model, MARKAL was implemented on a PC in 1990. The PC version is being rapidly adopted by existing MARKAL users worldwide. It has also opened the model to a wider community.