The adhesive strategies of marine mussels are key to their survival on wet, wind-swept, and wave-swept surfaces. Given this, mussel tenacity has become a poster child for the wet adhesion needed elsewhere in human technology, particularly in health-care delivery. Mussel adhesion is complex with both chemical and physical underpinnings at multiple length scales. The peculiar catechol-based chemistry of mussel adhesion has inspired a variety of applications ranging from hard and soft tissue repair to drug delivery to magnetic imaging agents. Although the emphasis on new bioinspired materials is inevitable, it should be coupled with the recognition that society is equally well served by the mussel byssus (holdfast) as an indicator of mussel well-being. Byssally interconnected mussel clusters are the basis of mariculture and diverse reef-like intertidal ecologies that resist coastal erosion. Given its exquisite sensitivity to environmental conditions, mussel byssus also serves as an important monitor of pollution and climate change.