  We present an extensive photometric and spectroscopic study of two
high-redshift clusters of galaxies based on data obtained from the Keck 10m
telescopes and the Hubble Space Telescope. The clusters CL0023+0423 (z=0.84)
and CL1604+4304 (z=0.90) are part of a multi-wavelength program to study nine
candidate clusters at z > 0.6 (Oke, Postman & Lubin 1998). Based on these
observations, we study in detail both the field and cluster populations. From
the confirmed cluster members, we find that CL0023+0423 actually consists of
two components separated by ~2900 km/s. A kinematic analysis indicates that the
two components are a poor cluster with ~3 x 10^{14} solar masses and a less
massive group with 10^{13} solar masses. CL1604+4304 is a centrally
concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226
km/s and a mass of ~3 x 10^{15} solar masses. Approximately 57% and 50% of the
galaxies show high levels of star formation in CL0023+0423 and CL1604+4304,
respectively. These numbers are significantly larger than those found in
intermediate redshift clusters. We also observe many old, red galaxies. Found
mainly in CL1604+4304, they have spectra consistent with passive stellar
evolution, typical of the early-type galaxies in low and intermediate-redshift
clusters. We have calculated their ages by comparing their spectral energy
distributions to standard Bruzual & Charlot evolutionary models. We find that
their colors are consistent with models having an exponentially decreasing star
formation rate with a time constant of 0.6 Gyr. We observe a significant
luminosity brightening in our brightest cluster galaxies. Compared to BCGs at z
< 0.1, we find a luminosity increase of ~1 mag in the rest B-band and ~0.8 mag
in the rest V-band.
