In this chapter, we characterise the structure of ego networks in Twitter. First, we describe the large-scale dataset that we collected from Twitter specifically for this part of the analysis. Then, we present an analysis of the structural properties of the Twitter ego networks extracted from the dataset. The analysis is similar to the one performed on Facebook, and uses the same methodology. We also present a comparison between Facebook and Twitter ego networks, to understand similarities and differences between the two social platforms, and thus to derive baseline properties of online social network environments in general.