The detection of gravitational waves from the mergers of binary black holes by the Advanced LIGO detectors has provided a first glimpse on the most energetic phenomena in the Universe. We can now look ahead and conceive a new generation of detectors with higher sensitivity. Europe has taken first steps years ago with the design study of the Einstein Telescope. The LIGO community in the US is discussing a similar project currently under the name Cosmic Explorer. In this talk, I will present the main challenges and constraints of next-generation detectors, and give examples of how the envisioned sensitivity gain would further improve our understanding of the Universe.