We discuss alternative techniques for microholography of biological specimens including requirements and constraints on the optical elements and recording media. We derive spatial and temporal coherence requirements for four representative holographic techniques and relate coherence and recording-medium resolution to maximum specimen volume. We give estimates of coherence length necessary to image a variety of specimens under a realistic set of assumptions. We discuss matters of wavelength selection with emphasis on the problem of obtaining high contrasts for nitrogen-bearing biological constituents. We use the eikonal approximation to derive equations for diffractive holographic imaging with particular attention to specimens of low contrast, and use these to illustrate the benefits of using 27r9 resonances to image molecular structures.