This paper presents an overview of our current understanding of the radiative properties of clouds, placing particular emphasis on recent results and unanswered problems arising from the marine stratocumulus and cirrus cloud components of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE), conducted in the United States during 1986 and 1987. For marine stratocumulus clouds, we present and discuss the discrepancy between observations and theory of the absorption of solar radiation by clouds, the discrepancy between remote sensing and in situ estimates of the effective droplet radius derived from spectral reflectance measurements, and the variability and spatial structure of stratocumulus clouds derived from both reflection and transmission measurements. We will describe the thermal emission characteristics of cirrus clouds and will demonstrate how the brightness temperature difference in the split-window region of the thermal infrared can be used to infer the effective radius of ice crystals, observations that lead to the conclusion that ice crystals are much smaller than previously believed. We will also illustrate the relationship between thermal emittance and visible albedo that has been derived from airborne observations of cirrus clouds. These results generally show that the thermal emittance of cirrus clouds is less than theoretically predicted for a given value of the visible albedo.