Aggregation concerns the conditions under which several variables can be treated as one, or macro-relationships derived from micro-relationships. This problem is especially important in production, where, without proper aggregation, one cannot interpret the properties of the aggregate production function. The conditions under which aggregate production functions exist are so stringent that real economies surely do not satisfy them. The aggregation results pose insurmountable problems for theoretical and applied work in fields such as growth, labour or trade. They imply that intuitions based on micro variables and micro production functions will often be false when applied to aggregates.