Time Domain Reflectometry (TDR) has recently become popular as a method to measure soil water content. An empirical Topp′s equation and some dielectric mixing functions are often used as calibration curves of TDR. This review describes the Topp′s equation and dielectric mixing models. In addition, applicability of Topp′s equation and some dielectric mixing models for the θ-ε relationships for typical upland soils (two types of Ando soils, Red-yellow soils, two types of Brown forest soils, and Toyoura sand) are discussed. Following are several remarks that are stressed on TDR: (1) The empirical Topp′s equation underestimates soil water content for mineral soils of low bulk density. (2) Since dielectric mixing models described the effect of bulk density on the calibration curve, they could be more suitable than the Topp′s equation for the experimented soils. (3) Judging from the fitness of the whole curve, the third-order polynomial regressions and α model were superior to the Maxwell-De Loor model, which has no parameter to be identified. (4) The Maxwell-De Loor model was found to be so flexible that it reasonably fitted with measured data for different types of soil.