For understanding the universe from gravitational wave astronomy, multi-wavelength observation will be important, analogous to EM astronomy advanced over the preceding decades. Multi-wavelength observations will be enabled with a variety of detection methods, including ground-based or space-borne interferometers and pulsar timing arrays. In particular, space experiments will open up a new range of frequencies, 10-7-10-1 Hz, for the observation of gravitational waves. They will allow us to study various gravitational wave physics in the unexplored parameter space of strain and frequency and will benefit the field, producing complementary results to further our knowledge on gravitational waves. We review briefly the detection of low-frequency gravitational waves, citing detailed references.