The quantity and quality of the adipose tissue is altered in obesity: hypertrophy of adipocytes, decrease in expandability, fibrosis, production of pro-inflammatory cytokines and adipokines, inflammatory infiltration by pro-inflammatory M1 macrophages and T lymphocytes. Excess adipose tissue and obesity are associated with low-grade inflammation that can aggravate chronic inflammatory diseases, decrease treatment efficacy, contribute to the development of comorbidities, and of the disease in its preclinical phase. Obesity increases the incidence of rheumatoid arthritis (RA), psoriasis, psoriatic arthritis with a weight-dose effect. It is also a predictor of worse response to treatment, in particular TNF inhibitors. Weight loss induced by bariatric surgery decreases the risk of psoriasis and psoriatic arthritis and improves disease activity and remission in RA. A 5% weight loss through a low-calorie diet improves the response to TNF inhibitors in psoriatic arthritis, losing even more weight improves the response even more. In RA, the beneficial effect of weight loss from a low-calorie diet on disease activity has been reported in a few observational studies but has not been demonstrated in a randomized controlled trial. Screening and management of obesity are part of the dietary recommendations of the French Society for Rheumatology in patients with chronic inflammatory rheumatic diseases. Of course, this should be combined with other lifestyle modifications such as smoking cessation, higher food quality, increased physical activity and reduced sedentary behavior.