The structure and behavior of packet switched networks is difficult to model in a way comparable to many natural and artificial systems. Nonetheless, the Internet is an outstanding and challenging case because of its incredibly fast development, unparalleled heterogeneity and the inherent lack of measurement and monitoring mechanisms in its core conception. In short, packet switched networks defy analytical modeling. This chapter is intended to introduce and provide concise descriptions of some of the building blocks of what some authors call Internet Science [21, 104], i.e., the study of laws and patterns in Internet structure. Additional related aspects that will be used throughout the next chapters are discussed as well. We will briefly define and describe the most relevant concepts about Internet performance and measurement that will be used throughout the next chapters. However, we will not get into details about all the networking concepts this monograph deals with.We refer to [37] for a good overall and in-depth analysis of traffic measurement and performance analysis. There are also a number of research papers that provide good insight into more specific topics. Among these, we highlight [21], where some key mathematical concepts in Internet traffic analysis are discussed. It is also out of the scope of this monograph to analyze in detail the mathematical aspects of most of the concepts this monograph deals with, and in particular those related to traffic control. For this, we refer the interested reader to [153] and [15]. Some of the most relevant and seminal research papers in this area can also be consulted [134, 132, 129, 171, 71]. © 2011 Springer-Verlag Berlin Heidelberg. Peer Reviewed