The public sphere was both one of the most popular and most debated concepts in political philosophy in the 20th century. It is one of the few concepts that has attracted attention from a wide range of disciplines. Despite the already rich intellectual history of the concept, many scholars still disagree over its proper use and definition. The purpose of this article is to review recent research into the public sphere with a co-citation analysis. A citation analysis helps to obtain a broader view of the development of the concept in different disciplines. Such an approach is of importance because the concept of the public sphere is neither fixed nor stable, as is typical for an essentially contested concept. Rather than finding the most prominent or even “right” interpretation of the concept, the primary goal of this study is to explore the evolvement and differentiation of the concept in different domains and disciplines over time. In such an analysis, the whole corpus of academic literature focusing on the public sphere as a concept can be captured. In a first step, I introduce the concept of the public sphere as an essentially contested concept. In a second step, I analyze 5386 publications from the last 20 years with a co-citation analysis. Based on the data, I identify different research communities, analyze the development of the communities and topics over time, and empirically observe some of the conditions of an essentially contested concept.