This article identifies six main theories of the determinants of social trust, and tests them against survey data from seven societies, 1999-2001. Three of the six theories of trust fare rather poorly and three do better. First and foremost, social trust tends to be high among citizens who believe that there are few severe social conflicts and where the sense of public safety is high. Second, informal social networks are associated with trust. And third, those who are successful in life trust more, or are more inclined by their personal experience to do so. Individual theories seem to work best in societies with higher levels of trust, and societal ones in societies with lower levels of trust. This may have something to do with the fact that our two low trust societies, Hungary and Slovenia, happen to have experienced revolutionary change in the very recent past, so that societal events have overwhelmed individual circumstances.