Classical and recurrent novae are among the most luminous transients in the Galaxy, powered by a thermonuclear runaway on an accreting white dwarf. Their eruptions probe extreme regimes of nuclear burning, mass ejection, radiative feedback, and shock physics, and they play a relevant role in the Galactic chemical evolution. X-ray emission from novae arises from three different components: the hot white dwarf atmosphere, visible as a super-soft source at the Eddington luminosity for some time after the outburst; the shock-heated plasma within different velocity components of the ejecta; and finally, when the outburst is over, the accretion-powered cataclysmic variable that has hosted the nova event. High-resolution X-ray spectroscopy uniquely constrains white dwarf temperatures and surface composition, while also diagnosing shock velocities, ionization states, and plasma temperatures in the expanding ejecta. Observations to date reveal complex, rapidly evolving spectra that challenge static atmosphere and simple shell models. Recent high-resolution X-ray spectroscopy shows radiative recombination continua (RRC) and signatures of charge exchange (CE)  in the expanding ejecta. At the same time, short-period oscillations detected in the super-soft emission of the post-nova remain a mystery, with their possible scenarios foreseen so far, related to g-mode pulsations or magnetic white dwarfs, both challenging our present understanding of the H-burning process in novae. On the harder X-ray band, shocks in the ejecta probed by X-ray spectroscopy are also responsible for the particle acceleration that powers the VHE detected in many novae. Finally, the effect of the nova outburst itself on the accretion disk and the inter-outburst evolution of the system has been only recently explored thanks to the eROSITA All Sky Survey, confirming the predicted evolution by multi-outburst simulations for the first time.