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Devices relying on the simultaneous manipulation of the electron spin and charge to perform their functions form the foundation of the spintronics. Compared to traditional CMOS technology, spintronic devices operate with lower power dissipation, at higher switching speed and would allow enhanced integration for logic operation and information storage. In the first part of this thesis, one of the basic building blocks of spintornics, the Datta-Das Spin Field Effect Transistor (SpinFET) was studied in great detail by removing the idealized assumptions made in the original proposal. First, it was shown that 100% spin injection efficiency is possible at a non-ideal ferromagnetic/semiconductor interface at a temperature of 0 K. Even when the ferromagnetic contacts are non-ideal with less than 100% spin polarization, the spin injection efficiency from the source into the channel can be switched from nearly +100 % to nearly -100 % with a small swing in gate bias. We also studied the conductance of the SpinFET with half-metallic ferromagnetic contacts and showed the presence of Fano resonances in the transmission spectrum. Finally, a new dual-gate SpinFET was proposed which can be switched from ON to OFF with just few mVs difference between the two gate voltages. This results in exceedingly low dynamic power dissipation during switching.Next, a non-equilibrium Green function (NEGF) approach was used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC). A small difference of potential between the two side-gates leads to an inversion asymmetry between the LSOC on both edges of the channel triggering a spontaneous spin polarization in the QPC. In the regime of single-mode transport, the spontaneous spin polarization can reach near 100% when the effects of exchange are taken into consideration and is accompanied by near perfect conductance polarization. This leads to the observation of a 0.5 (2e2/h) conductance plateau without the need of any externally applied magnetic field. The spontaneous spin polarization in the QPC and the associated conductance polarization can be reversed by flipping the polarity of the source to drain bias or potential difference between the two side gates. These numerical simulations are in good agreement with recent experimental results on InAs based QPCs.The NEGF approach was also used to study in detail the ballistic conductance of asymmetrically biased side-gated QPCs for a wide range of QPC dimensions and biasing parameters. Various conductance anomalies were predicted below the first conductance plateau (2e2/h) which are due to spontaneous spin polarization in the narrowest portion of the QPC. The number of observed conductance anomalies was found to increase with increasing aspect ratio (length/width) of the QPC constriction. These anomalies are fingerprints of spin textures associated with either a leaky single qubit state or a leaky singlet state in the constriction of the QPC.Finally, it was shown that two QPCs in series could act as an all electrical spin valve. Because of the long spin relaxation length (100 μm at room temperature), GaAs was chosen as the channel material. When the effects of spin relaxation in the channel between the two QPCs are neglected, the ON/OFF conductance ratio of the spin valve is given by 1/(1 – α2), where α is the spin polarization of the conductance for a single QPC. The ON configuration corresponds to the case of identical asymmetric bias across each QPC. The OFF configuration is for the case of opposite polarity for the asymmetric bias across each QPC. It was found that, at T = 4.2K, the ON/OFF conductance ratio can be made as high as 105 by increasing the aspect ratio (length/width) of the QPCs forming the spin valve.