Plant health is now influenced not just by soil, nutrients, and climate but also by a microscopic ecosystem coexisting with them. The plant microbiome is an essential and unseen partner of plants, primarily located in the rhizospheres, phyllospheres, and endospheres, with a smaller presence in the spermospheres and carpospheres. The plant microbiome serves as a vital living barrier, functioning as an external immune system crucial for phytoprotection processes (interaction, anticipation, and response), occupying the plant niche prior to invaders, releasing antimicrobial bioactive compounds, and initiating internal defenses upon the onset of threats. The plant microbiome functions as a bio-intelligent interface between the host plant and its surrounding ecosystem, which serves as a critical agronomic mediator. This mediator mobilizes essential nutrients, such as nitrogen, iron, and phosphorus from soil and atmospheric sources, while also promoting plant growth through the biosynthesis of phytohormones and enhances plant resilience to pedoclimatic stresses. The plant microbiome has become a key component of scientific research due to its ecological and physiological functions, which have the potential to enhance the sustainability, productivity, and resilience of agricultural ecosystems. Maintaining it is essential for preserving plant health and a strategic investment in food security and agricultural sustainability. This chapter explains the concept and diversity of the plant microbiome and its fundamental importance for plant health. It focuses on innovative approaches to altering and improving plant microbiome functions, identifying factors that influence its balance, such as agricultural practices and climatic conditions.