Bitcoin, a protocol for a new permissionless decentralized digital currency hailed the arrival of a new application domain for computer science. Following Bitcoin's arrival, a series of innovations derived from the state of the art in several fields has been applied to cryptocurrencies, and has been slowly reshaping monetary and financial instruments on public distributed ledgers. It was soon clear however that Bitcoin and similar cryptocurrencies still require additional improvements. This challenging domain presents researchers in the field with new and exciting questions. I provide examples from two main research threads, related to the scalability of the protocol and to its underlying incentives.