Spines are ubiquitous in both plants and animals, and while most spines were likely originally used for defense, over time many have been modified in a variety of ways. These modifications may alter the mode of defense or may allow spines to serve entirely different functions. Here, we review the biomechanics underpinning defensive spine use and modification. In order to understand the biomechanics of spines as puncturing tools we will explore how both the gross morphology of a spine affects its puncturing ability, as well as more fine-scaled surface morphology. In addition to puncture, many spines also deliver toxins as a part of their defense. While these spines often depend on puncture as part of their function, the mode of toxin delivery will vary between organisms. In other cases defensive spines don’t have to puncture at all in order to defend the organism. The defensive action of these non-puncturing spines may still stem from interactions with the predator, or the spines may interact with the environment to deter predation. In many fish, for example, spines serve to make the animal larger and therefore more difficult to swallow. In other cases, organisms that live or hide in burrows and crevices, like sea urchins, may use spines to wedge themselves in place and prevent predator access. Finally, in some cases, while spines still serve to deter predators they also have been modified to serve secondary functions unrelated to predation or defense. For example, some cactus spines have modifications that help facilitate clonal reproduction. Alternately the hollow spines of hedgehogs both serve to deter predators and can also act as cushioning from falls and impacts.