With the emergence of environmental concerns and the awakening regarding animal treatment issues, the anthropocentric paradigm has begun to shift, causing many countries to review their position on the legal status of animals. Within the movement for animals, there are two mainly followed philosophical theories: the animal welfare perspective, which has Peter Singer as its leading author, and the animal rights theory, likewise known as the abolitionist movement, with Tom Regan as its central theorist. Utilizing the method of comparative analysis, this article seeks to analyze each author’s thought process and compare theories, contrasting each viewpoint’s moral and philosophical foundations and which principle each author has determined as most fundamental. The main differences between them will also be compared, as well as their conclusions and effects on society, with a particular focus on their influences on the Brazilian Federal Constitution of 1988.