This work studies massive long-period exoplanets, similar to Jupiter, orbiting nearby Sun-like stars in our galaxy. The main method used is Doppler spectroscopy, also known as the Radial Velocity method. This research is based on two observational programs: A follow-up of long-period exoplanet candidates observed by the ELODIE and SOPHIE spectrographs for over 20 years, and a follow-up of planetary systems containing a hot Jupiter and signals of an additional giant planet in a wider orbit, observed with the CORALIE spectrograph. The characteristics of each program are discussed, as well as the results found. Additionally, the possibility of these radial velocity signals being caused by stellar magnetic cycles is also discussed. When this is ruled out, the orbital parameters and characteristics of every system are estimated. Finally, this thesis explores the possible synergies between Radial Velocities and other exoplanet detection techniques such as Direct Imaging, Astrom