  This is a review article on the primordial black holes (PBHs), with
particular focus on the massive ones ($\gtrsim 10^{15}{\rm g}$) which have not
evaporated by the present epoch by the Hawking radiation. By the detections of
gravitational waves by LIGO, we have gained a completely novel tool to
observationally search for PBHs complementary to the electromagnetic waves.
Based on the perspective that gravitational-wave astronomy will make a
significant progress in the next decades, a purpose of this article is to give
a comprehensive review covering a wide range of topics on PBHs. After
discussing PBH formation as well as several inflation models leading to PBH
production, we summarize various existing and future observational constraints.
We then present topics on formation of PBH binaries, gravitational waves from
PBH binaries, various observational tests of PBHs by using gravitational waves.
