Logical reasoning has been argued to crucially depend on linguistic or symbolic representations. This suggests that animals are incapable of negation. Nevertheless, animals have been found to behave in ways that suggest reasoning by negation. This chapter discusses the findings from the animal cognition literature in light of theoretical accounts of negation based on propositional and non-propositional thought. Instead of engaging in negation proper, animals might engage in proto-negation, that is reasoning based on contrary pairs instead of negated propositions. While most of the current findings might be explained in terms of proto-negation, an accumulation of evidence might eventually render negation proper a more parsimonious explanation of the evidence. The chapter concludes with suggestions for future avenues of research and a discussion of the role of negation in the evolution of human reasoning.