People are increasingly using multiple computing devices in their daily lives as portals into a shared online information space. We can select devices based on their form factor and affordances to match our task and context of use. Moreover, we are often using multiple devices at once, for example when sharing content, playing games, controlling other devices such as a smart TV, or collaboratively editing and presenting content in a meeting. The seamless use of multiple devices to work toward achieving the same goal is known as cross-device interaction. Cross-device interaction opens up new opportunities for how we engage with digital technology, but it also presents several fundamental challenges.