This article proposes that the groundbreaking concept of “criminal selectivity” (Zaffaroni 2000, 2011, 2011b), not yet extensively used in the English literature, could benefit from a critical conceptualization and operationalization. The hope is that the resulting tools might facilitate the identification of the unfairness patterns and the subsequent transformation of the criminal justice system unequal functioning. To do so, this contribution will explore the framework of the notion of criminal selectivity to later describe how it displays at the two stages of the criminal justice process, i.e., primary and secondary criminalization. On the one hand, “primary criminalization” involves the enactment of statutes by legislators and the executive power. On the other hand, “secondary criminalization” encompasses the enforcement of the law by police officers; the court process by prosecutors, defense lawyers, judges, and juries; and the administration of punishment by correctional officers or parole boards. As none of these instances can possibly hold all the possible negative social behaviors accountable, an inquiry arises: what are the criteria to criminalize some behaviors and not others? Digging into this question, the article will provide some conceptual precision in relation to the mechanisms through which criminal selectivity operates. It will be argued that this filtering process works disregarding some behaviors and actors (under-criminalization) and focusing on others (over-criminalization) and that this different attitude responds to socio-economic grounds rather than to the involved social harm. Finally, building upon the described conceptual tools, the article analyzes the criminal justice system in Argentina.