This chapter explores the small world phenomenon, which we often encounter in daily life when we find connections with strangers or discover many mutual friends on social networks. This phenomenon, also known as the six degrees of separation, has been extensively studied and successfully applied in platforms like LinkedIn. This chapter will cover the key characteristics of the small world phenomenon, including short paths, discoverability, and optimal distance. It will also review the primary methods of studying this phenomenon: experimental findings, model interpretations, simulation proofs, and empirical evidence from big data. Through an examination of various well-known small world models, this chapter will discuss the small world phenomenon in social networks and conclude with an analysis of the small world phenomenon in World Wide Web links.