In this thesis we investigate the static, dynamic, and thermodynamic propertiesof atomic spin-1 Bose gases in external magnetic fields. At low magnetic fieldsthe properties of single-component, or scalar condensates, are essentiallyunaffected but can become significantly altered for spinor Bose condensates asshown by our studies.We first study the Bose-Einstein condensation of trapped spin-1 Bose gases byemploying the Hartree-Fock approximation and the two-fluid model within a meanfield approximation. Our detailed investigation reveals that theferromagnetically interacting spin-1 condensates exhibit triple condensationswhile the antiferromagnetically interacting ones show double condensations.The ground state structure of homogeneous and trapped spin-1 Bose condensateswith ferromagnetic and antiferromagnetic interactions at zero temperature inmagnetic fields are then investigated systematically. We further illuminate theimportant effect of quadratic Zeeman shift which causes a preferred occupationof the $|m_F=0angle$ state through spin exchange collisions, $2|m_F=0angleleftrightarrow |m_F=1angle + |m_F=-1angle$.We also present detailed studies of the off-equilibrium coherent dynamics ofspin-1 Bose condensates in magnetic fields within the single spatial modeapproximation. Dynamical instabilities of the off-equilibrium oscillations areshown to be responsible for the formation of multiple domains as recentlyobserved in several $^{87}$Rb experiments.Finally, we discuss briefly excited condensate states, or soliton-like states,in cigar-shaped spin-1 Bose condensates with an effective quasi-1D description,using the developed nonpolynomial Schr"odinger equation.
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