For many years, the atomic structure of metallic glasses eluded a simple physical description. Inspired by the efficient filling of space with atoms of different sizes, a model has been developed by constructing efficiently packed atomic clusters and then efficiently filling space with these clusters. This model gives a simple physical picture of the origin of short-range and mediumrange order in metallic glasses, and also gives quantitative agreement with measurements that span length scales from nearest-neighbor atoms to global properties. A brief outline of the development and application of the efficient cluster packing model is given. Recent modifications to this model and remaining questions that form the basis of continued work are described.