In this chapter, we explore why toxin-producing algae—from cyanobacteria to dinoflagellates—invest so heavily in crafting complex and often deadly molecules. Moving beyond their reputation as mere agents of destruction, we examine the ecological functions of phycotoxins as tools of survival, competition, and adaptation. Through a synthesis of recent advances, this chapter disentangles how these compounds shape aquatic communities by eliminating competitors and predators, mediating species interactions, and influencing trophic dynamics. We begin by revisiting the role of cyanotoxins, with a focus on microcystins, whose ecological functions are increasingly understood in the context of competition, signaling, and self-regulation. We then turn to marine phycotoxins, from domoic acid to saxitoxin, prymnesins, and oxylipins, to illustrate their diverse roles in predator deterrence and interspecific competition. In doing so, we discuss how toxin production can be induced by grazers, revealing a dynamic evolutionary arms race within aquatic food webs. Emerging evidence from studies suggests that harmful algal blooms (HABs) are not simply ecological catastrophes but may also act as drivers of adaptation, shaping community structure, biodiversity, and even spawning cues in higher trophic levels. Building on this, we explore the underappreciated interactome—the coevolutionary partnerships between toxigenic algae and their associated microbiomes—which may hold the key to understanding toxin regulation and ecological success. Finally, we highlight the importance of adopting a systems-level perspective, treating toxigenic algae, their bacterial partners, and their grazers as interconnected holobionts. Integrating multi-omics approaches—from transcriptomics to epigenomics—offers the next frontier in revealing how these chemical dialogues sustain ecological balance and drive evolutionary innovation. By the end of this chapter, the reader will see phycotoxins not simply as poisons, but as agents of ecological negotiation—molecules that have helped shape aquatic life over evolutionary time.