In the present work, I focus on micro-industrial organization's and political economy's theories used in the study of media economics, referring to two particular streams of the recent literature: media as two-sided market (I.O. field) and the effects of the presence of media bias (political economy field). I provide three different essays on the issue of media markets. In the first part, I deal with the I.O. analysis of media as two-sided markets: the first chapter provides a general framework of vertical differentiated media, analysing the effects of real and potential competition; the second work extented the setting of the first, by comparing different markets structures from a welfare point of view. Finally, in the second part, I move ...