  We present a long-term multi-wavelength light curve of Galactic black hole
candidate GRS 1758-258 by combining previously published and archival data from
GRANAT, ROSAT, CGRO, RXTE, SAX, ASCA, EXOSAT, and the VLA. In addition we
include first spectral results from the balloon-borne Gamma-ray Arcminute
Telescope Imaging System (GRATIS). In light of divergent analyses of the
1991-1993 ROSAT observations, we have re-analyzed these data; we find the soft
X-rays track the hard X-rays, and that the fits require no black-body
component-- indicating that GRS 1758-258 did not go to the high state in 1993.
We offer an interpretation based on the ADAF model for a system with mdot
\ltsim mdot_crit. We find the 1990-1993 coeval hard and soft X-ray observations
support the ADAF predictions. We discuss a new way to constrain black-hole mass
with spectral data and the ADAF theory, and apply this technique to GRS
1758-258 to find M_1 \gtsim 8--9 M_sol at an assumed distance of 8.5 kpc.
Further investigations of the ADAF model allow us to evaluate the model
critically against the data and flux-flux diagram of Barret, McClintock &
Grindlay (1996) and to understand the limits of the latter's ``X-ray burster
box.''
