Numerous experimental and theoretical studies have been performed on flat plate solar collectors, but relatively little effort has been applied to investigating the manner in which they age. We have exploited the long-time and extensive use of solar energy for domestic hot water (DHW) heating in Israel to study how aging affects the physical conditions and thermal behavior of collectors. The scope of this study was limited to individual thermosiphonic DHW solar systems in the Tel-Aviv region and to collectors constructed from galvanized steel absorber plates and pipes. The study included visual inspections, owner interviews, thermographic analysis of collectors, standard efficiency tests, thermosiphonic efficiency tests, experiments relating degradation to performance, and dissection of solar collectors, including longitudinal sectioning of pipes, to observe the condition of the various internal components. Two of the major conclusions from this study are: (1) well made solar collectors can operate satisfactorily for periods in excess of 15 years with minimum maintenance. (2) Many of the more serious problems found were either present at the time of the installation (i.e. manufacturing defect) or resulted from poor installation techniques.