Medical, psychological and sociological research into the so-called «bridge populations» that link the core infected group with a wider population during an epidemic has always been considered difficult. However, it gets notoriously tricky when it comes to the HIV bridge populations. Given the current rate of the spreading epidemic in Eastern Europe, it is extremely important to teach the researchers to conduct scientifically valid and representative studies of these groups. An Arena CS-organised 4 day seminar in Tbilisi was single most important step in this direction this year, with professors from Universities of California and Zagreb sharing their experience with Ukrainian experts.