There are two basic types of fluid closure schemes. In truncation schemes, high-order velocity-space moments of the distribution function are assumed to vanish, or are prescribed in terms of low-order moments [24,36]. Truncation schemes are relatively straightforward to implement, but the error associated with the closure cannot easily be determined. Asymptotic schemes, on the other hand, depend on a rigorous expansion of the kinetic equation in terms of some dimensionless parameter that is small compared to unity . Asymptotic closure schemes have the advantage of providing some estimate of the error involved in the closure. However, the asymptotic approach to closure is mathematically demanding, because it involves working closely with the kinetic equation. In this book, we shall rely on a mixture of truncation and asymptotic closure schemes.