Laser speckle limits the resolution that can be achieved in holography. Time-varying speckle, caused by scattering from moving objects, is even more troublesome. It can destroy the correlation needed to obtain fringes in holographic interferometry and even, ifthe fluctuations are rapid compared with the exposure time needed, the coherence needed to record a hologram. These problems are discussed, with particular reference to biomedical holography, and suggestions are made for minimizing their effect. Some positive aspects of time-varying speckle are also discussed. The fluctuations obey the same statistics as the spatial variations of a stationary speckle pattern. These statistics can be used to measure the movement of the scattering particles, a technique of particular value in biomedical science. Potential applications include monitoring blood flow, motility, and intracellular activity.