We have used three automatic photoelectric telescopes to obtain photometric observations of 187 G, K, and (a few) M0 field giants. We find low-amplitude photometric variability on timescales of days to weeks on both sides of the coronal dividing line (CDL) in a total of 81 or 43% of the 187 giants. About one-third of the variables have amplitudes greater than 0.01 mag in V. In our sample the percentage of variable giants is a minimum for late-G spectral classes and increases for earlier and later classes; all K5 and M0 giants are variable. We also obtained high-resolution, red wavelength spectroscopic observations of 147 of the giants, which we used to determine spectral classifications, vsini values, and radial velocities. We acquired additional high-resolution, blue wavelength spectra of 48 of the giants, which we used to determine chromospheric emission fluxes. We analyzed the photometric and spectroscopic observations to identify the cause(s) of photometric variability in our sample of giants. We show that the light variations in the vast majority of G and K giant variables cannot be due to rotation. For giants on the cool side of the CDL, we find that the variability mechanism is radial pulsation. Thus, the variability mechanism operating in M giants extends into the K giants up to about spectral class K2. On the hot side of the CDL, the variability mechanism is most likely nonradial, g-mode pulsation.