Diet and physical activity seem to affect the course of disease and symptoms of patients with inflammatory bowel disease (IBD), including Crohn’s disease and ulcerative colitis. All available evidence regarding effects of diet and physical activity in IBD is in line with the general Dutch dietary and physical activity guidelines. In this thesis, those guidelines have been investigated specifically in IBD. Better adherence to the guidelines appears to be associated with less abdominal complaints. Furthermore, moderate-intensity physical activity, an important part of the physical activity guidelines, seems to be safe for IBD patients. By improving adherence to the guidelines, patients experienced less fatigue and impact of disease on daily life. An online tool can support IBD patients to improve their dietary intake. To summarize, this thesis shows that IBD patients benefit from optimizing their diet and level of physical activity according to the Dutch guidelines.