A Neutron Star is an extremely compact star with a mass of 1-2 solar masses and a radius of only 10 - 20 km. In general, there is an upper limit for the mass of a Neutron Star. But a fast rotating Neutron Star can be heavier than a slow rotating one. Due to its rotation the Neutron Star is deformed, its circumference is larger at its equator. Additionally, the rotation causes a moment of inertia I and a so-called quadrupolmoment Q. Moreover, there are the Love numbers, which describe the deformability of a Neutron Star. Between the moments of inertia, the quadrupolmoments, and the Love numbers there are important relations. They are called: I-Love-Q.