Introduction. Bacterial cells in the human body account for 1-3% of total body weight and are at least equal in number to human cells. Recent research has focused on understanding how the different bacterial communities in the body (eg, gut, respiratory, skin, and vaginal microbiomes) predispose to health and disease. For nearly 50 years, existing dogma has dictated that normal human endometrium is sacrosanct from microbial habitation in the absence of infection. However, while the vaginal microbiota has been investigated in depth, there is a paucity of consistent data regarding the existence of an endometrial microbiota and its possible impact in reproductive function. Aim of the study. To summarize the evidences derived from international studies on endometrial microbiome, its composition and potential influence in fertility and reproductive outcomes. Materials and methods. This study is a systematic review of data of the characteristics of endometrial microbiome in women with infertility and its association with the cause of infertility, conception rates and early pregnancy loss. Systematic literature searches of the electronic databases: Pubmed, EMBASE, the Cochrane library, MEDLINE, INTECH were performed up to February 2018 and included 44 studies. Studies were included if they reported on, at least, one of the following: characterization of endometrial microbiome in infertility – 30 studies, association with conception – 18 studies, with early pregnancy loss – 15 studies. Results. The most represented genus was Lactobacillus (71.7% of identified bacteria); while Gardnerella (12.6%), Bifidobacterium (3.7%), Streptococcus (3.2%), and Prevotella (0.866%) were the other most common genera. Based on its composition, the microbiota in the endometrial fluid, comprising up to 191 operational taxonomic units, was defined as a Lactobacillus-dominated microbiota - >90% Lactobacillus spp., or a non-Lactobacillus-dominated microbiota - <90% Lactobacillus spp. with >10% of other bacteria (Bacteroides, Prevotella, Fusobacterium, Atopobium vaginae, Mobiluncus curtisii). The presence of a non-Lactobacillus-dominated microbiota in a receptive endometrium was associated with significant decreases in implantation [60.7% vs 23.1% (P=.02)], pregnancy [70.6% vs 33.3% (P=.03)], ongoing pregnancy [58.8% vs 13.3% (P=.02)], and live birth [58.8% vs. 6.7% (P=.002)] rates. Conclusions. The human microbiome has been termed the second genome, and its importance in reproductive success and failure has yet to be fully appreciated. The reviewed studies demonstrated the existence of an endometrial microbiota that is highly stable during the acquisition of endometrial receptivity. However, pathological modification of its profile is associated with poor reproductive outcomes.