Hi Dotdottung, in my university, I call those with associate professor as 'Dr. X' but those with professor title, I address them as 'Professor X'. Actually, the 'associate professor' title in my university is a promotion for those who were previously senior lecturers/assistant professors. How you address them I think also depends on the organization culture and your relationship with them I think, e.g. those who I am friendly with don't mind being called "Dr" as opposed to "Prof".... :-)