This collection features five peer-reviewed reviews on biostimulant applications in agriculture. The first chapter provides a comprehensive overview of the optimal design and formulation of microbial and non-microbial biostimulants. The chapter presents two case studies to demonstrate the successful commercialisation of microbial and non-microbial biostimulant products. The second chapter considers the utilisation of humic substances (HS) as plant biostimulants in agriculture and their impact on the physical and chemical properties of soil. The chapter also discusses the production of HS. The third chapter reviews the wealth of research on the mechanism of action, applications and efficacy of key biostimulants such as Bacillus species, Pseudomonas species, Trichoderma species and arbuscular mycorrhizal fungi. The fourth chapter assesses recent advances in the use of plant growth-promoting rhizobacteria (PGPR) as a means of enhancing crop root function and nutrient use. The chapter also considers the challenge of practical adoption and use of PGPR in commercial agricultural settings. The final chapter reviews the utilisation of biostimulants as an integrated pest management tool in horticulture. The chapter considers their role in promoting plant growth, building soil structure, as well as pest and disease suppression.