Interpretations of depositional environments from microfossils are based mainly on comparisons of fossil assemblages with the micro-organic remains in present day sea muds. Qualitative comparisons of species, genera, and families common to both shales and sea muds have provided two broad generalizations. First, benthonic organisms are the chief indicators of temperature, depth, amount of light, water composition, and other bottom conditions. Second, pelagic, planktonic, and nektonic organisms overlap into a variety of sedimentary environments indicating only broad latitudinal boundaries of temperature and salinity. Quantitative studies of micropopulations provide the best data from which to reconstruct the environments of deposition. Furthermore, quantitative data give promise of being new stratigraphic correlation tools, their importance ranking with lithologic and electrical logs. Five important basic principles apply to microfossils if they are employed to determine environmental conditions. First, microfossils are integral parts of the lithology of shales. Since the specimens are particles of sediments, then the fossils and the lithology should be considered a closely knit result of the whole environment. Second, specimens are derived simultaneously from the living (biocoenosis), the dead debris (thanatocoenosis), and agents of sedimentation (wave action, currents, turbidity currents, mud slides, and organisms). Third, in the processes of sedimentation, gravity moves micro-organic remains to deeper locations on the ocean floor. Consequently, the upper depth and temperature limits of living organisms are the keys to depth and temperature interpretations. Fourth, the use of biological ecology data for comparisons with microfossil suites brings in only the biocoenosic elements and omits the results of sedimentation. Fifth, geographic distribution and coordination of each microfossil suite to the paleogeographic setting are necessary to reconstruct the environment.