Extragalactic X-ray studies provide unique opportunities for studying accreting black holes. In particular, they are necessary for studying phenomena not easily selected or observed in other wavelengths. Among these objects, ultra-luminous X-ray sources (ULXs) emit the vast majority of their luminosity in the X-ray band and are very faint or confused in other wavebands. Similarly, heavily obscured active galactic nuclei (AGN) with absorbing columns > 10^{24} cm^{-2} are rarely detected in optical surveys, due to the extreme reddening. In my thesis, I study both phenomenon in the local universe.

At ULX luminosities [L_X (0.3 - 10 keV) > 3 x 10^{39} erg s^{-1}], the European Space Agency's XMM-Newton satellite provides the spectral resolution and sensitivity necessary to study the population of local ULXs. Thus, we conducted an XMM-Newton archival study of a complete sample of the ULXs located within 8 Mpc. Our study confirmed key predictions of the intermediate mass black hole (IMBH) hypothesis for local ULXs. We then followed-up this study by investigating high signal-to-noise XMM-Newton observations of 14 ULX sources - studying their spectral shape, testing the validity of different accretion disk and power law models, and then using absorption of their spectra to measure the oxygen and iron abundances of the interstellar medium of their host galaxies.

New breakthroughs are expected in the study of heavily obscured AGN from SWIFT. The SWIFT satellite, launched in 2004, has detected a sample of 153 AGN with the Burst Alert Telescope (BAT) in the first 9-months of data. The BAT is sensitive in the 14 - 195 keV band and the selected sources have an average redshift of ~ 0.03. Thus, it detects local AGN without bias towards all but the most obscured sources (n_H > 10^24 atoms cm^-2). The BAT AGN sources are the result of an all-sky survey with a flux limit of F_BAT > 10^{-11} erg s^{-1} cm^{-2}.
I analyze and present the results of X-ray data from XMM-Newton, ASCA, as well as SWIFT's XRT (0.3 - 10 keV) and BAT, in order to understand the properties of obscured and unobscured AGN in the local universe. Among our results, we show that the new class of ``hidden''/buried AGN are a significant population of local AGN (~ 20%). We also find that our data supports the need for a modified AGN unified model -- one which includes a luminosity dependence.