The indoor environment affects the health, comfort and performance of occupants. All three are important, but the cost of improved design, operation, maintenance and energy use in buildings must usually be justified in terms of the expected effects on productivity. While there is no doubt that effects on health, comfort and performance translate into effects on productivity, there is an urgent need to discover which of the hypothesised mechanisms of cause and effect are valid to quantify how much each mechanism affects productivity in different work situations, and to validate predictions in the field.