The information on the causes of forest fires is of paramount importance to support the environmental and civil protection policies and design appropriate prevention measure. At the European level a simple common scheme with 4 fire causes classes (deliberate, accident/negligence, natural and unknown) has been used to record information on fire causes since 1992. European countries use national schemes which in most cases are much more detailed than the simple 4 common classes, but they are not harmonized and detailed cross country comparisons are difficult. The need for a new EU scheme, more detailed than the 4 basic categories and harmonized across European countries, to be recorded in the Fire Database of the European Forest Fire Information System (EFFIS), has been identified to improve the information level and the common knowledge on the origin of forest fires in Europe. The new scheme has been conceived to be applicable with limited changes to the previous country settings, preserving as much as possible the historical data series of each country and exploiting at the same time as much as possible the level of detail of the information available. This report provides a detailed description of the new scheme, its main features with precise definition of each cause class and the mapping of the historical national systems to the new harmonized system. The scheme is hierarchical and is made of 29 fire cause classes, 8 groups and 6 categories. The explicit statement on the level of certainty in the attribution of the cause to a fire event has been introduced as a key element in the new scheme. This harmonized fire cause classification scheme is expected to be adopted by the countries participating to the EFFIS network in the coming years, and therefore be recorded in the European Fire Database, with a significant added value for the knowledge about the origin of forest fires in Europe.