Caltech has no Greek life. There are no sororities or fraternities. However, the housing system resembles that of the Greek system. The houses are setup in a way that they are coed frats with their own traditions and initiations. Even the way new members are acquired each year resembles the Greek bidding system, as the first week of school when the freshmen visit each house (a process known as Rotation) serves as the equivalent of rushing.
However, the housing system does differ from a Greek system in many ways. For one, membership is not necessary. Students are required to go through Rotation and are placed into a house; they later drop membership if they are not pleased with the house or system. This does not happen very often, though. Also, membership in more than one house is fairly common. Some students who start hanging out with people in other houses may eventually want to join that house. Each house has its own way of admitting new members. Overall, the housing system is pretty awesome and provides a support structure for students here.