The discovery of quasars in the early 1960's quickly spurred the idea that these amazingly powerful sources derive their energy from accretion of matter onto a compact, extremely massive object, most likely a supermassive black hole (SMBH; Zel'dovich & Novikov 1964; Salpeter 1964; Lynden-Bell 1969) with M 106 - 109 M. Since then this model has provided a highly useful framework for the study of quasars, or more generally, of the active galactic nucleus (AGN) phenomenon (Rees 1984; Blandford & Rees 1992). Yet, despite its success, there is little empirical basis for believing that this model is correct. As pointed out by Kormendy & Richstone (1995, hereafter KR), our confidence that SMBHs must power AGNs largely rests on the implausibility of alternative explanations. To be sure, a number of characteristics of AGNs indicate that the central engine must be tiny and that relativistic motions are present. These include rapid X-ray variability, VLBI radio cores, and superluminal motion. However, solid evidence for the existence of SMBHs in the centers of galaxies has, until quite recently, been lacking.
As demonstrated by Soltan (1982), simple considerations of the quasar number counts and standard assumptions about the efficiency of energy generation by accretion allows one to estimate the mean mass density of SMBHs in the universe. The updated analysis of Chokshi & Turner (1992) finds 2 × 1050.1-1 M Mpc-3 for a radiative efficiency of = 0.10.1. Comparison of with the B-band galaxy luminosity density of 1.4×108h L Mpc-3 (Lin et al. 1996), where the Hubble constant H0 = 100h km s-1 Mpc-1, implies an average SMBH mass per unit stellar luminosity of ~ 1.4×10-3 0.1-1h-1 M / L. A typical bright galaxy with LB* 1010h-2 L potentially harbors a SMBH with a mass 1070.1-1 h-3 M. These very general arguments lead one to conclude that "dead" quasars ought to be lurking in the centers of many nearby luminous galaxies.
The hunt for SMBHs has been frustrated by two principal limitations. The more obvious of these can be easily appreciated by nothing that the "sphere of influence" of the hole extends to rh GM / 2 (Peebles 1972; Bahcall & Wolf 1976), where G is the gravitational constant and is the velocity dispersion of the stars in the bulge, or, for a distance of D, ~ 1" (M / 2 × 108 M)( / 200 km s-1)-2(D/5 Mpc). Typical ground-based observations are therefore severely hampered by atmospheric seeing, and only the heftiest dark masses in the closest galaxies can be detected. The situation in the last few years has improved dramatically with the advent of the Hubble Space Telescope (HST) and radio VLBI techniques. The more subtle complication involves the actual modeling of the stellar kinematics data, and in this area much progress has also been made recently as well.
Here I will highlight some of the observational efforts during the past two decades in searching for SMBHs, concentrating on the recent advances. Since this contribution is the only one that discusses nuclear BHs aside from that in the Milky Way (Ozernoy, these proceedings) and in NGC 4258 (Miyoshi, these proceedings), I will attempt to be as comprehensive as possible, although no claim to completeness is made, as this is a vast subject and progress is being made at a dizzying pace. To fill in the gaps, I refer the reader to several other recent review papers, each of which has a slightly different emphasis (KR; Rees 1998; Richstone 1998; Ford et al. 1998; van der Marel 1999).