The idea of holography in string theory provides a simple geometric computation of entanglement entropy. This generalizes the well-known Bekenstein-Hawking formula of black hole entropy and strongly suggests that a gravitational spacetime consists of many bits of quantum entanglement. After we give a brief review of this field, I will explain progresses on a recently introduced quantity called pseudo entropy, which generalizes the entanglement entropy and has a manifest gravity dual. We will also discuss an extension of this idea to holography for de Sitter spaces.