This paper presents the results of an ergonomic assessment of all relevant factors influencing the performance of air traffic controllers in the aerodrome, area and approach control in Croatia Control in Zagreb. Measured values of individual ambient factors (air temperature, relative humidity, illumination, and equivalent noise level) in Croatia Control in Zagreb, were compared with the results of online questionnaire self-assessment of disturbance factors and needs of air traffic controllers throughout the Republic of Croatia as well as with the guidelines from recent professional and scientific literature, and with ICAO Recommendations. Recent rules have been applied to ensure that equipment and other elements of the working environment need to be adjusted to the specific needs of users. Ergonomic assessment of all factors that can affect the performance and safety of air traffic controllers includes factors of workload, the factors of shift organization which include night shifts, and design factors of the working environment. In accordiance with disadvantages in design of working environment which have been located, guidelines for possible solutions have been suggested.