The discovery of GW150914 showed some black holes do merge and the coalescence of a binary black hole can be "directly observable" with gravitational waves (GWs). Although imaging is not possible with GW observation, masses of individual black holes in a binary can be measured by GW observation with impressive accuracy. The distance to the source is also measurable and BBHs are suggested as 'standard sirens' for constraining a Hubble constant. Stellar-mass BBHs are observable with km-scale laser interferometry in the GW frequency range of 10-2000 Hz. Supermassive BBHs would be observable by precision timing of millisecond radio pulsars at nano Hertz frequencies of GW signals. In this talk, I will give an overview on the second Gravitational-Wave Transient Catalog (GWTC-2) published in October, 2020 by the LIGO-Virgo collaboration and the 11-yr efforts of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) published in 2018. I will also discuss the implications of BBH observations for the next decades.