In frozen soils, a portion of pore water remains unfrozen due to the effects of capillarity, adsorption, and possibly solute. The variation of the amount of unfrozen water and ice in a frozen soil, which is primarily influenced by subzero temperature, has great impacts on the physical and mechanical behavior of the soil and is critical for broad applications ranging from engineering to climate change. In the present study, the various methods that have been used for determining unfrozen water (and ice) content are comprehensively reviewed. Their principles, assumptions, advantages, and limitations are discussed. It is noted that there is yet no perfect way to accurately quantify unfrozen water content in frozen soils. In addition, the soil-freezing characteristic curve (SFCC) of a typical volcanic soil sampled in the Hokkaido prefecture of Japan is investigated. The unfrozen water content of the prepared soil specimens was measured using a cheap moisture sensor, which is based on the frequency domain reflectometry technique. The temperature of the specimens was determined by a rugged temperature sensor. Different numbers of freeze-thaw (F-T) cycles and different freezing/thawing methods (i.e., one- and three-dimensional) were considered, and their effects on the SFCC were investigated. The experimental results suggest that neither the F-T cycles nor the freezing/thawing methods had significant influence on the measured SFCC. The presented comprehensive review and experimental investigations are of importance for both the scientific and engineering communities.