One of the predictions of General Relativity are the gravitational waves. It was announced in February 2016 that the gravitational waves were finally observed for the first time in the history. It means that we obtain completely new window for the observation of the Universe. Before that event we observed the Universe almost only in electromagnetic radiation. But we have only theoretical models, what happened in the first second of the evolution of the Universe, because we are not able to detect any radiation from this epoch of evolution. But if we are able to detect primordial gravitational waves, this situation will change. Gravitational waves were already observed at LIGO and VIRGO terrestrial observatories. People prepare so called LISA observatory in space, it will be prepared for launch around 2029. And the next generation of observatories are BBO, which will be already able to detect primordial gravitational waves. I try to explain in this lecture, how we describe gravitational waves in modified gravities.