  Direct observations of gravitational waves at frequencies around deci-Hertz
will play a crucial role in fully exploiting the potential of multi-messenger
astronomy. In this chapter, we discuss the detection landscape for the next
several decades of the deci-Hertz gravitational-wave spectrum. We provide an
overview of the experimental frontiers being considered to probe this
challenging regime and the astrophysics and fundamental goals accessible
towards them. This includes interferometric observatories in space with
heliocentric and geocentric satellites, cubesats, lunar-based experiments and
atom intereferometry. A major focus of this chapter is towards the technology
behind DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) and
its scientific pathfinder mission concept B-DECIGO.

General Relativity and Quantum Cosmology;High Energy Astrophysical Phenomena