The new world 800m champion Caster Semenya has been asked to undergo gender tests by the International Association of Athletics Federations after finishing significantly faster than the other competitors in the women's race.
It may be thought that determining if someone is a man or woman would be as simple as looking to see if they have breasts and a vagina or a penis.
But in reality it is far more complex.
Even someone's external genitalia can be "ambiguous". For example, the clitoris may be enlarged so that it looks like a small penis or a female's labia may be fused, resembling a scrotum.
There are also chromosomal and hormonal variations and conditions which medics will test for.
The South African athlete will be assessed and tested by a group of doctors, including an endocrinologist, a gynaecologist, an internal medicine expert, an expert on gender and a psychologist.
They will look at her external genitalia, but they will also look at hormone levels and her chromosomal make-up.
There are also particular conditions they will check for, including congenital adrenal hyperplasia.
This is a condition in which the body produces more androgen, a type of male hormone.
If a girl has it, she will usually have normal internal female reproductive organs, but may not have periods and may have a male appearance.
This, and a number of other conditions, are recognised by the IAAF as potentially giving some advantage but are accepted.
There are other conditions, including polycystic ovaries and androgen producing tumours, where a woman can have higher than normal levels of testosterone which are not thought to offer any advantage to athletes.
In addition, there is a condition called androgen insensitivity syndrome where someone may have internal and undescended testes - and high levels of testosterone - but look like a woman and have a vagina and a uterus.
Professor Adam Balen, a specialist at Leeds General Infirmary, said: "This is an extremely complicated area.
"The reality is that the difference between males and females with respect to strength is based upon hormones.
"So if you have high levels of testosterone, you are more likely to have better muscle strength."
Such conditions are rare. Around one in 20,000 UK children is born with some kind of sexual development disorder.
However rates can vary around the world, usually because of genetic variations.