Angular momentum of a supermassive black hole (SBMH) is a vitally important quantity in astrophysics. Measuring the spins of SMBHs in active galactic nuclei (AGN) can inform us about the relative role of gas accretion vs. mergers in recent epochs of the life of the host & its AGN. Black hole spin is also thought to play a pivotal role in triggering relativistic jets, enabling the SMBH to influence its surroundings out to scales much larger than its gravitational sphere of influence allows. Advances in theoretical modeling as well as observational sensitivity in the Chandra/XMM-Newton/Suzaku era are finally producing robust constraints on the spins of a handful of SMBHs, as well as physical properties of the accretion disk & structure of the AGN as a whole. This science is still very much in its infancy, however. I will discuss our current knowledge of the distribution of SMBH spins in the local universe, w an emphasis on addressing complexities involved in using X-ray spectroscopy to obtain these measurements. I will also address prospects for improving accuracy, precision & quantity of these spin constraints in the next decade & beyond with instruments such as NuSTAR, Astro-H & ATHENA. Additionally, I will demonstrate the importance of considering spectral variability on a wide variety of timescales when modeling the emission & absorption properties of the AGN system. Proper modeling of this variability can yield critical insights into the physical structure of the nucleus.