Magnonics is an exciting and rapidly growing field revolving around the study and manipulation of magnons, the low-lying collective excitations of magnetically ordered systems. This field has emerged in response to both fundamental physics interests and the growing demand for faster, more efficient, and more reliable signal processing and computation up to THz frequencies, i.e., beyond clock frequencies of current computer technology. Magnonic devices promise to transmit and process information in ways that are fundamentally different from traditional electronic devices that exploit the flow of charges. Low-frequency spin waves can propagate through magnetic materials with minimal energy loss, transmit information via angular momentum flow instead of charge motion, and can be easily manipulated using magnetic fields, electric fields, spin currents, or thermal gradients. High-frequency magnons offer wave-based in-memory computation at wavelength much shorter than light, contributing to the emerging request for beyond von Neumann computer architectures. Consequently, numerous efforts are focused on developing spin-wave-based approaches to information processing that encode information in the amplitude and phase of spin waves and manipulate it via spin-wave gates and spin-wave interferometers. Magnons also give rise to the new burgeoning field of hybrid magnonics, which aims at leveraging the interactions between magnons and other degrees of freedoms to unlock unprecedented functionalities and physical regimes. In particular, the integration of magnons with other quantum systems, such as superconducting circuits, quantum dots, or nitrogen-vacancy centers in diamond, leads to several advantages for quantum information processing and quantum sensing and opens new avenues for research into the quantum properties of magnons.