This article briefly describes the basic concepts involved in the two most commonly used molecular modelling methods-molecular dynamics (MID) and Monte Carlo (MC). The methods are particularly useful for studying structures at the length scale of nanometre. Two examples (both are on the study of the miscibility of polyolefin blends) are used to illustrate the techniques. It is demonstrated that it is the nano-scaled structures formed by the segments of the constituent polyolefins that prevent them from mixing with each other. The examples also show that selection of specific method (MID or MC) depends on the nature of the problem in hand. In general, MC is more efficient than MID in terms of generating equilibrated structure while MID can provide information about the dynamics of a system. This is simply because MID requires the solution of equations of motion (a set of second order differential equations) while MC does not. Nonetheless, both methods need a reasonably accurate force field.