Airspace capacity in a high-density air traffic network is determined by controller workload. Controller workload is primarily affected by the features of the air traffic and air traffic control (ATC) sector. Factors that affect controller workload are examined by, first, a review of the literature on this subject and then a multivariate analysis of the data obtained from simulations of a model of air traffic controller workload. This model, the European Airspace Model (EAM), has been widely used in European airspace planning. The results from simulations of five very different areas of European airspace are analyzed. In particular, the results of principal components analysis and factor analysis provide a valuable insight into the aircraft profile factors affecting controller workload.