Abstract: An important class of microscale fluid-structure interactions in biology involve the interactions and deformations of flexible elasticae, both passive and active, with fluid flows. This is evident from fundamental biological transport processes such as the swimming microorganisms using internally actuated cilia or flagella, the transport of material by the coordinated action of ciliary carpets, and the involvement of both actuated and passive flexible filaments in the first symmetry-breaking of vertebrate cells. Both single filaments, and assemblies, are challenging to study because individual filaments have many internal degrees of freedom in deformation and can exhibit microscopic instabilities. The interaction of many filaments in these assemblies can be manifested as macroscopic emergent behavior. I'll tell you two stories concerning the dynamics of passive and active fibers in flows. In the first, I'll discuss the novel buckling instabilities and complex shapes of single actin polymers in simple flows. In the second, I'll tell you how beds of beating cilia organize themselves to convert the nanoscopic action of internal molecular motors into the large-scale metachronal waves that underlie pumping and clearance.