The general features of elastic behavior, brittle failure, and plastic yielding are investigated under plane stress for a material composed of arrays of elastic-brittle or elastic-plastic filaments. Filamentary material linear elastic constitutive relations are derived and conditions for planar elastic isotropy are explored. Initial and subsequent fracture surfaces (which are generally anisotropic) and stress-strain behavior are computed for brittle filament materials. The form of incremental plastic stress-strain laws, initial yield surfaces, and subsequent yield surfaces are presented for general filament work-hardening. Examples are presented for brittle, perfectly plastic, and kinematic linear work hardening filaments, illustrating application of the analysis and the main results.