Oral and systemic human health depend on the symbiotic relationship between the human host and its microbiome. As the second most diverse site of the human microbiome, the oral cavity is instrumental in symbiotic relationships, transforming nutrients and acting as the human body's initial barrier against pathogens. However, under certain conditions, the typically beneficial oral microbiome can become harmful. Systemic inflammatory diseases can send signals through the oral-gut axis, such as cytokines and host defensins, altering gene expression and, consequently, the composition of the oral microbiome. These changes can be responsible for causing oral diseases, such as periodontitis and candidiasis. Evidence of metabolic syndrome, including obesity, hypertension, hyperglycemia, and dyslipidemia, exacerbates oral microbiome dysbiosis. On the other hand, the oral microbiota can also influence systemic health. Inflammatory processes in the gingival structures caused by a dysbiotic oral microbiome are linked to worsen glycemic levels in diabetics, premature birth, and rheumatoid arthritis, among others. The idea for this book emerged from the need to explore the multifaceted nature of this relationship in its various dimensions. We discuss multispecies characteristics from an ecological perspective, focusing on how the host affects the microbiome and vice versa. Understanding how the oral microbiome influences human health will guide tailored strategies for disease prevention and treatment, which is discussed in the last section of the book. Looking ahead, predictive health and disease models will enable personalized therapies centered on restoring the healthy human microbiome.