Solar collectors are the most important part of the solar thermal energy conversion system because they determine the amount of energy collected and absorbed in order to be used for a certain application. Accurate design and simulation of solar collectors allow proper selection and efficient use of such devices according to the available solar resource, required performance parameters (level of temperature and heat quantity), land use and budget. In addition, assessment of the analysis of performance of solar collectors enables the use of new configurations with better performance. This book introduces an investigation of performance of different types of solar thermal collectors for the use in residential, commercial and industrial applications. These collector types are investigated according to the temperature level that can be obtained by utilizing proper geometrical and operating conditions, low temperature (with no or low concentration) and medium temperature (with medium concentration). A generalized mathematical model for optical and thermal behavior of solar thermal collectors is developed and applied to the different collector types.