The goal of performance modeling is to gain understanding of a computer systems performance on various applications, by means of measurement and analysis, and then to encapsulate these characteristics in a compact formula. The resulting model can be used to gain greater understanding of the performance phenomena involved and to project performance to other system/application combinations. The focus here, as in other chapters in this volume, is on large-scale scientific computation, although many of the techniques we describe apply equally well to single-processor systems and to business-type applications.