Researchers studying digital communication typically focus on a single communication channel. However, research on media multiplexity shows that strong relationships tend to involve communication across a variety of channels, such as text messages, voice calls, social media, and in-person communication. This chapter identifies more specifically which dimensions of closeness are associated with media multiplexity by disentangling communication frequency, cognitive closeness, and social role (e.g., being family or co-workers). It uses a unique data set that combines behavioral indicators of logged texting and calling with self-report measures. The results show that high communication frequency, cognitive closeness, and an important social role are all independently associated with media multiplexity.