Compositions and methods for the in situ formation of cross-linked polymeric filaments are disclosed. The disclosed filaments are preferably elastomeric, water-resistant, oil-resistant, long-wearing. The filament forming compositions generally comprise one or more cross-linkable polymers having at least one functional group capable of forming cross-links and a cross-linking agent, the presence of which promotes cross-linking of the polymers. The compositions may be used for lengthening and/or volumizing keratin fibers, for example, eyelashes.