This paper systematically reviews scientific papers to understand the primary forms of HI, why they arise, and how they affect the urban landscape. It addresses the question, “What are the key trends, drivers, and impacts of Housing Informality (HI), and what methods researchers use to study the latter?”. Previous research addressed the trends, drivers, impacts, and methods individually, failing to adopt a holistic perspective that could clarify HI as a core scientific concept distinct from informal housing. Accordingly, this systematic literature review (SLR) explores 541 scientific peer-reviewed publications from Scopus, Web of Science, and Google Scholar. By utilizing a PRISMA framework, we reduced the number of documents to 27, which we thoroughly analyzed. The results highlight five main trends of HI around the globe: unauthorized modifications, informal market dynamics, Accessory Dwelling Units (ADUs), squatting, and unusual housing. These trends are induced by several economic and sociocultural drivers that address the gap between regulations and governance from one side and lived realities from another, resulting in economic, social, legal, health, and urban impacts.