The goal of this lecture is to inform the audience of the types of challenges that arise, and open problems that remain, in a specific class of biological fluids: mucus. Mucus is prevalent in biology and its rheology is fundamental for: locomotion (e.g., of snails); flow transport (e.g., of mucosal layers in mammalian lungs); and controlling diffusive transport of invasive particles (e.g., in the nasal cavity, lung, and reproductive organs). Mucus varies dramatically across species, across populations, across organs, and in a single organ across disease states. The lecture will address challenges faced in the Virtual Lung Project at UNC for design of experiments, for data-based inference of constitutive parameters, and for development of direct simulation tools for lung biology and medical applications.