Excluding parenteral transmissions, virtually all vertical and homosexual transmission of human immunodeficiency virus type 1 (HIV-1) occurs via the gastrointestinal tract. Cellular routes implicated in the translocation of virus across the epithelium include M cells, dendritic cells, and epithelial cells. Intestinal epithelial cells express CCR5 and can selectively transfer CCR5-tropic HIV-1, the phenotype of the majority of transmitted viruses. In the lamina propria, virus encounters the largest reservoir of mononuclear cells in the body. Surprisingly, lamina propria lymphocytes, not macrophages, express CCR5 and CXCR4 and support HIV-1 replication, implicating intestinal lymphocytes as the initial target cell in the intestinal mucosa. From the mucosa, virus is disseminated to systemic sites, followed by profound depletion of CD4+ T cells, first in the intestinal lamina propria and subsequently in the blood. As mucosal and circulating CD4+ T cells are depleted, monocytes and macrophages assume an increasingly important role as target and reservoir cells for HIV-1. Blood monocytes, including HIV-1-infected cells, are recruited to the mucosa, where they differentiate into lamina propria macrophages in the presence of stroma-derived factors. Although the prevalence of HIV-1-infected macrophages in the mucosa is low (0.06% of lamina propria mononuclear cells), the extraordinary size of the gastrointestinal mucosa imparts to intestinal macrophages a prominent role as a HIV-1 reservoir. Elucidating the immunobiology of mucosal HIV-1 infection is critical for understanding disease pathogenesis and ultimately for devising an effective mucosal HIV-1 vaccine.