The field of microbial endocrinology is expressly devoted to understanding the mechanisms by which the microbiota (bacteria within the microbiome) interact with the host (“us”). This interaction is a two-way street and the driving force that governs these interactions are the neuroendocrine products of both the host and the microbiota. Chapters include neuroendocrine hormone-induced changes in gene expression and microbial endocrinology and probiotics. This is the first in a series of books dedicated to understanding how bi-directional communication between host and bacteria represents the cutting edge of translational medical research, and hopefully identifies new ways to understand the mechanisms that determine health and disease.