The presentation includes several examples of application of holography to measurements of the population of particles in space. A reconstructed hologram is a frozen, three-dimensional image of a sample volume that can be scanned at different magnifications. Consequently, it is possible to identify and determine the spatial distribution of particles and droplets in air as well as bubbles, particles, and plankton in water. Multiple-exposure holograms can be used for velocity measurements. Due to these advantages, holography has been used extensively for particle detection. The disadvantages have been the cumbersome optical setup and the tedious analysis procedures, but recent developments in image processing offer some remedies. The presentation includes examples of implementation of holography to measure the bubble and particle populations in water, both in the laboratory, while studying cavitating flows, and in the ocean, including two generations of submersible systems. The minimum particle diameter that can be resolved in liquid is 10 (mu)m using in-line holography and 3 (mu)m using a hybrid, off-axis system. The maximum particle density varies between 1 and 5 per mm[sup 3], depending on the optical setup. Recent application to precision, 3-D, high-density (10[sup 6] vectors per hologram) velocity measurements will be demonstrated.