The u-invariant arises naturally from the study quadratic forms since it gives a uniform bound on the number of variables n such that any quadratic forms of n variables are universal. Particularly, u-invariant is the minimal such n satisfying such property. In this talk, I’ll give a survey of some known results in the u-invariant. The main goal is to compute u-invariants of some standard fields and discuss the relationship of u-invariants between field extensions.