Obesity in men of reproductive age is globally on the increase. There is clear evidence from epidemiological studies that obesity impacts negatively on male fertility; it is associated with hypogonadism, although it is less consistently linked to impaired spermatogenesis and tests of sperm function, including DNA fragmentation. Sperm from obese men used for in vitro fertilisation/intra cytoplasmic sperm injection is associated with a greater number of pregnancy losses and is less likely to result in live births. There are also increasing data from animal studies that paternal obesity may impact negatively on the reproductive and metabolic health of offspring and grand-offspring. It has been suggested that high-fat dietary exposures could affect the epigenetic content of sperm or the endocrine content of seminal fluid and thus impact early fetal development. Experimental and epidemiological data show that male fertility, and offspring health, can be improved by weight loss in obese and overweight males.