In this dissertation I present a study of the geometry and energetics of bundles composed of flexible cohesive filaments. This is a general class of materials, both biological and artificial, existing across many length scales. The aim of this thesis is to investigate the interdependence between the 2D organization of filaments in a bundle’s cross section, and the 3D structure, with an emphasis on the twisting mode of deformation. First we present a model of filament contacts and interactions, which we employ in numerical simulations to study the connection between the ground state energies of constant-pitch bundles and their interior packing topology. We then focus on exterior features, and construct a continuum model of the surface energy and its twist-dependence. Finally, we employ a fully 3D model of filament bundles with a fixed packing topology to examine the connection between filament organization and the resulting 3D structures, be they twisting, writhing, bending, undulating, or other modes of deformation.