Soil harbors the greatest biodiversity on the planet, and soil biodiversity plays a significant role in maintaining carbon dynamics and nutrient cycling in terrestrial ecosystems. Currently, soil biodiversity is one of the most important issues in soil ecology. In this review, we summarize the developments and achievements in soil biodiversity research over the last two decades. First, we introduce three critical developmental stages in soil biodiversity research, namely forums and symposiums, experimental testing, and monitoring and conservation practices. Then, we discuss four important soil biodiversity topics that have attracted extensive attention, and that epitomize important progress in soil biodiversity research. These topics include the mechanisms explaining The enigma of soil biodiversity, the relationships between soil biodiversity and ecosystem functioning, the biogeography of soil biodiversity, and the links between soil biodiversity and plant diversity. Finally, we discuss the challenges for future research on soil biodiversity, including the practical implementation of new methodologies, analyses of geographical distributions of soil biota, the impacts of human disturbance and global change on soil biodiversity, and the roles of soil organisms in ecological restoration.