This study reviews the fundamental mechanisms of biological soil improvement methods-microbially induced calcium carbonate precipitation (MICP) and biopolymer treatment (BPT). Extensive experimental data on various geotechnical properties of sands treated by MICP and BPT are compiled, including the unconfined compressive strength, Mohr-Coulomb shear strength parameters, and permeability. Furthermore, the variations in these engineering parameters are correlated to calcium carbonate content for MICP and biopolymer content for BPT, which provides insights into the extent of biological modification in engineering properties of sands, potential applications, and limitations.