Significant research has been conducted over the last two decades applying nondestructive ground-penetrating radar (GPR) measurements to evaluate the amounts of both fine-grained particles (fines) and moisture present in railway ballast. This has led to the development of a variety of quantitative as well as qualitative GPR signal interpretation techniques as variations in the amount of ballast fines and moisture result in detectable changes in recorded GPR signals. The accumulation of fines within the void spaces among the ballast particles decreases the size of the air voids while also altering the bulk dielectric permittivity and electrical conductivity of the ballast layer. Changes to the bulk ballast electromagnetic (EM) properties in turn affect GPR wave travel times, reflection strengths, and signal attenuation, while changes in air void size can directly impact the scattering behavior of GPR waves as they propagate through the ballast. Measurements from low-frequency (< 1 GHz) GPR systems are predominately affected by the changes in bulk EM properties, whereas high-frequency (> 1 GHz) GPR measurements, while affected by changes in signal attenuation, also respond directly to changes in air void size. The majority of GPR-based detection methods infer ballast degradation based on the resulting effects on the bulk EM properties; however, direct approaches, which do not rely on these intermediary properties, also exist. This paper presents a review of how ballast degradation, characterized by the accumulation of fines and increased moisture retention, affects the EM properties critical for GPR and contrasts the current methods used to estimate ballast conditions from GPR measurements.