This thesis was triggered by the question what is the nature of the sources producing the bulk of the X-ray background at hard energies. Of course the pre-Chandra missions did not allow any definite answer to this question. Therefore the approach adopted in this work is to examine whether candidate populations of objects could contribute to the X-ray background significantly. This was achieved by studying spectroscopically a variety of objects. To be more specific, in Chapter 3 it was examined whether broad line quasars, which produce the bulk of the X-ray background at soft energies, contribute significantly to the hard X-ray background as well. In Chapter 4 the X-ray properties of a sample of Seyfert 2 galaxies were examined. The results were examined in the light of the current X-ray background synthesis models. This study revealed two "odd" Seyfert 2 galaxies (NGC3147 and NGC4698). Odd, in the sense that although both galaxies are classified as Seyfert 2 in the optical, their X-ray data do not show evidence for absorption above the Galactic. Possible scenarios explaining the emission from the two galaxies are examined in Chapter 5. Chapter 6 considers the spectral properties of the Ultraluminous infrared galaxy 'The Superantennae'. In Chapter 7 a study of the composite galaxy Mrk609 is presented. The composite galaxies resemble the narrow-line X-ray galaxies (NLXGs) detected in large numbers in deep ROSAT surveys. Thus the study of the spectra of composite galaxies provides an indirect insight into the spectra of the NLXGs. Finally Chapter 8 summarises the results of this work.