  Active Galactic Nuclei (AGN) are known to show flux variability over all
observable timescales and across the entire electromagnetic spectrum. Over the
past decade, a growing number of sources have been observed to show dramatic
flux and spectral changes, both in the X-rays and in the optical/UV. Such
events, commonly described as "changing-look AGN", can be divided into two
well-defined classes. Changing-obscuration objects show strong variability of
the line-of-sight column density, mostly associated with clouds or outflows
eclipsing the central engine of the AGN. Changing-state AGN are instead objects
in which the continuum emission and broad emission lines appear or disappear,
and are typically triggered by strong changes in the accretion rate of the
supermassive black hole. Here we review our current understanding of these two
classes of changing-look AGN, and discuss open questions and future prospects.
