Vermicomposting is a low-technology, environmentally-friendly process used to treat organic waste. The resulting vermicompost has been shown to have several positive impacts on plant growth and health. This organic fertilizer is therefore increasingly considered in agriculture and horticulture as a promising alternative to inorganic fertilizers and/or peat in greenhouse potting media. However, the effects of vermicompost on plant-soil systems are not yet fully understood. In this chapter we summarize the research carried out during the last few decades, and the proposed mechanisms explaining the effects of vermicompost on soil quality and plant growth. Although much effort has been dedicated to the investigation of biologically mediated mechanisms of promoting plant growth, the conflicting results indicate the need to open up new lines of research, defining a clear and objective concept of vermicompost, and clarifying the conditions and sources of variability in the biological effects. A case study is presented in which the direct and indirect effects of vermicompost on plant growth, as well as variability in the plant responses, are examined in a field experiment with sweet corn.