  The diversity and quantity of moons in the Solar System suggest a manifold
population of natural satellites exist around extrasolar planets. Of peculiar
interest from an astrobiological perspective, the number of sizable moons in
the stellar habitable zones may outnumber planets in these circumstellar
regions. With technological and theoretical methods now allowing for the
detection of sub-Earth-sized extrasolar planets, the first detection of an
extrasolar moon appears feasible. In this review, we summarize formation
channels of massive exomoons that are potentially detectable with current or
near-future instruments. We discuss the orbital effects that govern exomoon
evolution, we present a framework to characterize an exomoon's stellar plus
planetary illumination as well as its tidal heating, and we address the
techniques that have been proposed to search for exomoons. Most notably, we
show that natural satellites in the range of 0.1 - 0.5 Earth mass (i) are
potentially habitable, (ii) can form within the circumplanetary debris and gas
disk or via capture from a binary, and (iii) are detectable with current
technology.
