Cloud optical depth is one of the most important cloud optical properties, and vital for any cloud-radiation parameterization. To retrieve cloud optical depth, the atmospheric science community has widely used shortwave flux measurements from both broadband and narrowband radiometers. Flux-based methods are, however, limited to overcast conditions and give, at best, "effective" values of cloud optical depth. Unlike flux observations, a method based on zenith radiance provides "local" estimates of cloud properties, and works even in a fully 3D cloud situation. We will present examples using data from the Atmospheric Radiation Measurement (ARM) program, the Micro-pulse Lidar Network (MPLNET), and the Aerosol Robotic Network (AERONET). Applications of our cloud property retrievals to study aerosol indirect effects will be also demonstrated.