Studies of radio-loud AGN are crucial to address basic questions like how jets are formed, accelerated and collimated, and what is the effect of the presence of such powerful jets in the evolution of the host galaxy and the local environment. In this thesis we have focused on two open issues relevant to the radio-loud AGN phenomenon, the nature of Giant Radio Galaxies (GRGs) and the AGN-host galaxy coevolution. We present a sample of 197 GRGs with a largest (projected) linear size (LLS) of at least 1 Mpc, divided into 93 low-z (z<0.4) and 104 high-z (z>0.4) sources for which optical spectra from the Sloan Digital Sky Survey (SDSS) are available. We obtained optical and radio parameters of GRGs in order to search for the reason for their having grown to Mpc scales. Moreover, we studied the circumnuclear regions of GRGs and 5 BL Lacs in order to get insights about the coevolution between the AGN and the host galaxy.