The microphysical aspects of the relationship between radar reflectivity factor Z and rainfall rate R are examined. The forward problem of analytically characterizing the Z-R relationship based on exponential, gamma, and monodisperse raindrop size distributions is highlighted as well as the inverse problem of a microphysical interpretation of empirically obtained Z-R relation coefficients, including the associated uncertainties. Many results reported in the literature are discussed within a coherent framework, providing new insights to issues evolving around the Z-R relationship. Three special modes that a Z-R relationship may attain are revealed, depending on whether the variability of the raindrop size distribution is governed by variations of drop number density, drop size, or a coordinated combination thereof with constant ratio of mean drop size and number density. The number-controlled case results in linear Z-R relations that have been observed for steady and statistically homogeneous or equilibrium rainfall conditions. Most rainfall situations, however, exhibit a variability of drop spectra that is facilitated by a mix of variation of drop size and number density, which results in the well-known power-law Z-R relationships. Only few rainfall conditions result in physically meaningful relationships between Z and R. Graphical ways to identify those situations will be presented.