We develop a theory that describes the properties of the recently discovered boron nanotubes. Our theory is based on a structure model of a broad boron sheet, being a single quasiplanar layer of boron. Based on the properties of that boron sheet, we propose a new route to achieve control over the atomic structure of nanotubes during their synthesis. Our results show that structure control can be accomplished by nanotubes which are rolled up from sheets with anisotropic in--plane mechanical properties. We then consider intramolecular junctions between carbon and boron nanotubes. The structural compatibility of the two classes of materials is shown, and a simple recipe that determines all types of stable linear junctions is illustrated. Our results suggest the existence of novel types of nanotubular compound materials, and point out the possibility of wiring nanotubular devices within heterogeneous nanotubular networks.We further study the high--pressure phase diagram of various bulk structures of boron. In particular, we investigate layered boron materials, which are a new family of hypothetical bulk phases which we regard as stacked arrangement of different broad boron sheets. These metallic materials are likely to exist at elevated pressures, or even at ambient conditions, and there are strong indications that they are conventional superconductors. Therefore, layered bulk phases of boron have the potential to explain the experimentally observed high--pressure superconductivity. Furthermore, we present the first realization of the generalized pseudoatom concept introduced by Ball, which we call "enatom". This enatom is calculated using numerical linear response methods, and the enatom quantities are analyzed for both fcc Li and Al at different pressures. Our results establish a method to construct the enatom, whose potential is to obtain a real--space understanding of solids, their vibrational properties and electron--phonon interactions.