Size-frequency distributions of volcanic tuffs precisely characterize several types of deposits and provide a means for evaluating the mechanism of transport and deposition. Particulate size analyses may be visually interpreted when plotted on arithmetic probability paper in phi notation. As with sedimentary clastic units, four statistical moments describe size distributions: phi mean, standard deviation, skewness, and kurtosis. Relationships between size-frequency distributions and transport energies exist for rhyolitic ash flow, rhyolitic air fall, rhyolitic base surge, basaltic air fall, and basaltic base-surge deposits. The three energy environments, which singly, or in combination control the grain-size populations of air-transported tephra, are: fallout, suspended load, and bed load. Pyroclastic deposits that closely approximate single-mode, log-normal distributions (or their skewed and kurtic variations) result from deposition in a predominantly single-energy environment. Polymodal distributions are mixed populations that represent transitions from one transport energy to another. The few tephra samples that have single-mode distributions closely approximating Rosin's law of crushing are ambiguous cases and may not have undergone enough transport to modify the initial grain-size distribution produced by volcanic explosion.