  Electromagnetic observations of the sky have been the basis for our study of
the Universe for millennia, cosmic ray studies are now entering their second
century, the first neutrinos from an astrophysical source were identified three
decades ago, and gravitational waves were directly detected only four years
ago. Detections of these messengers are now common. Astrophysics will undergo a
revolution in the 2020s as multimessenger detections become routine. The 8th
Astro2020 Thematic Area is Multimessenger Astronomy and Astrophysics, which
includes the identification of the sources of gravitational waves,
astrophysical and cosmogenic neutrinos, cosmic rays, and gamma-rays, and the
coordinated multimessenger and multiwavelength follow-ups. Identifying and
characterizing multimessenger sources enables science throughout and beyond
astrophysics. Success in the multimessenger era requires: (i) sensitive
coverage of the non-electromagnetic messengers, (ii) full coverage of the
electromagnetic spectrum, with either fast-response observations or broad and
deep high-cadence surveys, and (iii) improved collaboration, communication, and
notification platforms.
