IMPORTANCE: Inflammatory bowel diseases (IBD), such as Crohn disease and ulcerative colitis, are chronic relapsing conditions that affect a growing number of children worldwide. The pathogenesis of these disorders is complex and thought to be mediated by the interplay between genetic susceptibility, microbial dysbiosis, and environmental factors that result in a dysregulated immune system. This dysregulation ultimately mediates intestinal inflammation and clinical symptoms typically observed in patients with IBD including abdominal pain, diarrhea, and poor growth. A dramatic increase in the incidence of IBD has been observed in the past 2 decades, mainly in developed countries but also in developing regions. This increased incidence has paralleled changes in diet, sanitation conditions, and lifestyle habits. OBSERVATIONS: The increased incidence of IBD can likely be attributed to more than evolving genetic diversity alone and strongly suggests that environmental factors are playing an increasingly critical role in the development of these disorders and in the modulation of IBD clinical phenotypes over time. Here, we review the data suggesting how different environmental factors may modulate the risk of developing IBD including diets, smoking, lifestyle choices, enteric infections, appendectomy, air pollution, and the use of medications, with an emphasis on antibiotics. We will also discuss how early-life events can influence the subsequent likelihood of developing one of these diseases and suggest directions that can help decrease the risk of IBD in particularly high-risk populations. CONCLUSIONS AND RELEVANCE: Dramatic lifestyle changes in the last century have substantially improved the quality of life but are also associated with increased risk of various diseases. Pediatricians should be aware of the changing epidemiology of IBD and environmental factors that modulate the risk of developing these conditions.