Shaikh Al-Albaanee (rahimahullaah) said the following about pants (or trousers if you're from the UK):
"From these conditions is that the garment should not be tight such that it describes the ?Awrah (the part of the body that is private and it should be concealed from public view). And the ?Awrah of the woman of course, is larger (broader) than the ?Awrah of the man. So a condition is that the garment should not be descriptively tight. Pants, for example, it is not permissible to wear them from two aspects. The first aspect is that they are from the garments of the disbelievers. And the second aspect is that they describe the shape of the thighs, and the buttocks. Rather, they also describe what is between them as well, with much regret. This garment (pants) also contradicts the Islamic law."