Gravitational-wave observations in the 0.1 Hz band will open a unique observational
window between ground-based detectors and LISA. This talk begins with a brief review of
the scientific potential of gravitational-wave observations in this frequency range,
including searches for a primordial gravitational-wave background from inflation,
early-warning observations of binary neutron star mergers, and standard-siren
measurements for cosmology. I then discuss the key detector features required for such
observation in the 0.1 Hz band. While a shorter baseline than LISA can yield improved
peak strain sensitivity with the use of optical cavities, it also imposes significantly
more stringent requirements on displacement noise. Finally, I introduce recent
activities and future ambitions aimed at addressing these challenges, which are
universally important for realizing gravitational-wave observations in the deci-hertz
band.