Insulated glass solar thermal collectors result from the insertion of a solar absorber into the outer gap of a multiple glazed unit. Taking advantages from the manufacturing technology of the glass and window industry, a flexible and highly automated production as well as an easier and architecturally more appealing integration into the building envelope is expected. Ensuring long-term functionality, on the other hand, represents a very challenging development task due to high temperatures and to the thermally induced deformations of the solar absorber. The paper analyzes the behavior of this new kind of collector by means of theoretical calculations and experiments, focusing on both the performance and the reliability aspects. Efficiency measurements according to EN 12975 on prototypes with a slim design report specific values comparable to those of standard flat plate collectors (η0 = 0.78, a1 = 3.77 W/m2K, a2 = 0.011 W/m2K2 for a 50 mm thick, argon-filled prototype). Prolonged exposure to stagnation temperature and internal thermal shock-tests attest the durability of collector configurations featuring temperatureresistant components and suitable constructions, able to reduce and compensate the absorber deformation.