  The science achievable with SKA HI surveys will be greatly increased through
the combination of HI data with that at other wavelengths. These
multiwavelength datasets will enable studies to move beyond an understanding of
HI gas in isolation to instead understand HI as an integral part of the highly
complex baryonic processes that drive galaxy evolution.
  As they evolve, galaxies experience a host of environmental and feedback
influences, many of which can radically impact their gas content. Important
processes include: accretion (hot and cold mode, mergers), depletion (star
formation, galactic winds, AGN), phase changes (ionised/atomic/molecular), and
environmental effects (ram pressure stripping, tidal effects, strangulation).
Governing all of these to various extents is the underlying dark matter
distribution. In turn, the result of these processes can significantly alter
the baryonic states in which material is finally observed (stellar populations,
dust, chemistry) and its morphology (galaxy type, bulge/disk ratio, bars,
warps, radial profile). To fully understand the evolution of HI and the role it
plays in galactic evolution requires the ability to quantify each of these
separate processes, and hence to coordinate SKA HI surveys with extensive
multi-band photometric and spectroscopic campaigns. In addition,
multiwavelength data is essential for statistical methods of HI analysis such
as HI stacking and intensity mapping cross-correlations.
  In this chapter, we examine some of the principal science motivations for
acquiring multiwavelength data to match that from the extragalactic SKA HI
surveys, and review the currently planned capacity to achieve this (eg. LSST,
Euclid, W-FIRST, SPICA, ALMA, and 4MOST).
