Dense stellar systems and massive black holes : sources of gravitational radiation and tidal disruptions Gravity dictates the structure of the whole Universe and although it is triumphantly described by the theory of General Relativity it is the force that we least understand in nature. One of the cardinal predictions of this theory are black holes. Massive dark objects are found in the majority of galaxies. Our own galactic center very contains such an object with a mass of about four million solar masses. Are these objects supermassive black holes (SMBHs) or do we need alternatives The answer lies in the event horizon the characteristic that defines a black hole. The key to probe the horizon is to model the movement of stars around a SMBH and the interactions between them and look for deviations from real observations. Nuclear star clusters harboring a massive dark object with a mass of up to ~ ten million solar masses are good testbeds to probe the event horizon of the potential SMBH with stars. The channel for interactions between stars and the central MBH are the fact that (a) compact stars and stellar mass black holes can gradually inspiral into the SMBH due to the emission of gravitational radiation which is known as an “Extreme Mass Ratio Inspiral” (EMRI) and (b) stars can produce gases which will be accreted by the SMBH through normal stellar evolution or by collisions and disruptions brought about by the strong central tidal field. Such processes can contribute significantly to the mass of the SMBH. These two processes involve different disciplines which combined will provide us with detailed information about the fabric of space and time. In this habilitation I present nine articles of my recent work directly related with these topics.