These notes represent an extended version of the contents of the third lecture delivered at the AMS Short Course "Radon Transform and Applications to Inverse Problems" in Atlanta in January 2005. They contain a brief description of properties of some generalized Radon transforms arising in inverse problems. Here by generalized Radon transforms we mean transforms that involve integrations over curved surfaces and/or weighted integrations. Such transformations arise in many areas, e.g. in Single Photon Emission Tomography (SPECT), Electrical Impedance Tomography (EIT) thermoacoustic Tomography (TAT), and other areas. 1 Numerous other reasons to study this transform are known, e.g. Radar and Sonar imaging, approximation theory, PDEs, potential theory, complex analysis, etc. [2, 93]. Although in dimensions higher than two one should probably use the word "spherical" rather than "circular," we will use for simplicity the latter. This should not create any confusion.