In this thesis, nanostructures which may display novel spintronic behaviors are revealed and their properties are investigated by using first-principles methods. We have concentrated on three different systems, namely carbon linear chains, singe-wall carbon nanotubes and silicon nanowires. First of all, an extensive study of the electronic, magnetic and transport properties of finite and infiniteperiodic atomic chains composed of carbon atoms and 3d transition metal (TM) atoms are carried out. Finite-size, linear molecules made of carbon atomic chains caped with TM atoms, i.e. TM-Cn-TM structures are found to be stable and exhibit interesting magnetoresistive properties. The indirect exchange interaction of the two TM atoms through a spacer of n carbon atoms determines the type of the magnetic ground state of these structures. The n-dependent variations of the ground state between ferromagnetic (F) and antiferromagnetic (AF) spin configurations exhibit several distinct features, including regular alternations and irregular forms. We present a simple analytical model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. The periodically repeated TM-Cn atomic chains exhibit half-metallic properties with perfect spin polarization at the Fermi level (EF ). When connected to appropriate electrodes the TM-Cn-TM atomic chains act as molecular spin-valves in their F states due to the large ratios of the conductance values for each spin type. Secondly, a systematic study of the electronic and magnetic properties of TM atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs) is presented. The adsorption on the external and internal wall of SWNT is considered and the effect of the TM coverage and geometry on the binding energy and the spin polarization at EF is examined. All those adsorbed chains studied have F ground state, but only their specific types and geometries demonstrated high spin polarization near EF . Their magnetic moment and binding energy in the ground state display interesting variation with the number of d−electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic metal electrodes, these magnetic needles or nanomagnets can perform as spindependent resonant tunnelling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Finally, bare, hydrogen terminated and TM adsorbed Silicon nanowires (SiNW) oriented along [001] direction are investigated. An extensive analysis on the atomic structure, stability, elastic and electronic properties of bare and hydrogen terminated SiNWs is performed. It is then predicted that specific TM adsorbed SiNWs have a half-metallic ground state even above room temperature. At high coverage of TM atoms, ferromagnetic SiNWs become metallic for both spin-directions with high magnetic moment and may have also significant spin-polarization at EF . The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Most of these systems studied in this thesis appear to be stable at room temperature and promising for spintronic devices which can operate at ambient conditions. Therefore, we believe that present results are not only of academic interest, but also can initiate new research on spintronic applications of nanostructures.