Gravitational-wave interferometric detectors are very sensitive instruments which are, therefore, also subjected to a variety of noises. Among these, the Newtonian noise (gravity fluctuations noise) will affect the sensitivity curve at low frequencies. In this talk I will focus on this kind of noise, explaining what it is and the methods that we can use to reduce it. In particular, I will focus on the methodologies to actively cancel it, although other approaches are possible, which will also be quickly discussed.