Inflammatory bowel diseases (IBD), including Crohn's disease and ulcerative colitis, are featured by chronic intestinal inflammation, which is becoming increasingly prevalent in Western societies, and is spreading to the rest of the world. Although the etiology of IBD is poorly understood, it is widely accepted that several factors may be crucial, which includes genetics, diet and lifestyle, immunity, environment, and microbiota. In the past decade, huge advances have been made toward a better understanding of IBD, epitomized by far-reaching progress in the field of microbiome. In this review, we summarize the current knowledge of how changes in microbiota may affect the pathogenesis of IBD. Commensal bacteria, benign normally, are essentially opportunists that may readily take over, and potentially contributes to dysbiosis, which, in turn, promotes pathogenesis. Several pathogens, mainly Mycobacterium avium paratuberculosis, adherent invasive Escherichia coli, Clostridium difficile, Campylobacter, and Salmonella, have been shown to be associated with IBD, but the causality remains unproven. The microbiome comprises of not only bacteria, but also viruses, bacteriophages, and fungi. However, little is known as per the role of the latter. We highlight recent research on viruses, bacteriophages, and fungi in IBD. We also discuss the progress on manipulating the microflora to serve therapeutic purposes. The methodology for manipulating the microbiota covers fecal transplantation, pre-, pro-, syn-and post-biotics, helminth therapy, bacteriocins, bacteriophage, etc., which greatly enriches the arsenal against IBD. Manipulation of intestinal microbiome represents a promising type of therapy for IBD.