The Direct Simulation Monte Carlo method is a computational tool for modelling rarefied flows. The Macroscopic Chemistry Method was developed to simplify the modelling of dissociation and recombination reactions in DSMC. The ability to understand and predict the behaviour of chemically reacting, rarefied flows is a critical aspect in the development of high altitude, high speed bodies such as re-entry craft, high altitude aircraft, space transport vehicles and missiles. Computational methods are an invaluable source of information when experimental techniques are difficult, costly or time-consuming. However, traditional methods of modelling chemical kinetics using DSMC suffer from a number of drawbacks. The Macroscopic Chemistry Method overcomes a number of these problems, but has previously only been applied to simulations of a single diatomic gas. The Macroscopic Chemistry Method (MCM) is extended to consider multiple species and multiple reaction sets, thermal non-equilibrium effects, trace species modelling, unsteady flows, vibrational state specific chemistry, electronic excitation, relaxation and ionization and coupled nonequilibrium radiation emission. The Macroscopic Method is described as a general DSMC modelling philosophy rather than as a single formulated method. That is, the flexibility and utility of the method are shown through examples of applying a macroscopic approach to a number of problems, and by highlighting instances where a macroscopic approach is useful or even necessary. The problems investigated include reservoir relaxation calculations, 1-D shock, expansion and shock-expansion calculations, two-dimensional flows over a vertical step and through a cavity, and axis-symmetric flow about a sphere. The studies demonstrate that although MCM may often present a simplified approach as compared to traditional 'non-macroscopic' methods, it does not necessarily lead to more approximate solutions. On the contrary, the ability of macroscopic methods to combine different models of physical processes with the most recent (verified) data means that they are particularly suited to simulate high altitude, rarefied flows. It is also shown that, like any model approach, the validity of the approximations employed must be justified for a particular problem. In general, macroscopic methods of varying complexity and accuracy may be implemented to model a specific physical process. Adoption of the Macroscopic Chemistry Method in DSMC has the potential to enhance the modelling of chemical kinetics, charged-particle effects and radiation in rarefied hypersonic flows. This capability may be attributed to the simplicity and flexibility which the macroscopic approach affords over methods which seek to avoid the use of collective information. Macroscopic methods have already been employed to model weakly ionized flows. Their further application to model chemical kinetics and other processes would be useful for modelling and understanding the behaviour of objects in rarefied hypersonic flow-fields.