||Urban green spaces are essential natural environments for densely populated areas and offer refuges from the hectic city life and work environments. They constitute spaces that provide relatively low levels of social contacts, at the same time they are settings for social gathering. High-use levels as well as too low use levels in urban recreational areas may also be associated with negative effects due to overcrowding, safety concerns or too low social stimulation levels. Given the prominence of urban recreation areas in our daily life, it is surprising that so far rather little research has focused on the crowding perceptions and the social carrying capacities of urban park users, particularly in Europe. In eight different green spaces in Vienna, about 1700 on-site visitors were interviewed on randomly selected eight sampling days in 2006. Green spaces were heavily used small inner urban parks, various historical gardens and forests, and peri-urban recreation areas with a large area size and with low visitation. Interviews lasted between 15 and 20 minutes in most cases. One or two interviewers were used per study site. Crowding issues were asked in three separate questions using bi-polar measures: Visitors were asked about their crowding perceptions of the respective recreation area for both Sundays and workdays, using a 7-point scale ranging from a too lonely situation to an overcrowded situation (global measures of crowding). Actual crowding at the time of the interview was investigated using the same 7-point scale (actual measure of crowding). Crowding expectations and perceived development of use levels since the first visit were also asked. Overcrowding and undercrowding perceptions were expressed. About 47% of respondents expressed overcrowding perceptions for Sundays, while for workdays mostly pleasant crowding perceptions were reported. All crowding measures differed significantly, and significant differences were found across the green spaces. Research was supported by the Austrian Science Fund (FWF).