Magnetic nanoparticles are currently the focus of investigation for a wide range of biomedical applications that fall into the categories of imaging, sensing, and therapeutics. A deep understanding of nanoparticle magnetization dynamics is fundamental to optimization and further development of these applications. Here, a summary of theoretical models of nanoparticle dynamics is presented, and computational nonequilibrium models are outlined, which currently represent the most sophisticated methods for modeling nanoparticle dynamics. Nanoparticle magnetization response is explored in depth; the effect of applied field amplitude, as well as nanoparticle size, on the resulting rotation mechanism and timescale is investigated. Two applications in biomedicine, magnetic particle imaging and magnetic fluid hyperthermia, are highlighted. An overview of the magnetism dynamics of magnetic nanoparticles and its relevance to optimizing and further developing biomedical applications is presented, with a specific focus on the applications of magnetic particle imaging and magnetic fluid hyperthermia. A summary of historical and current theoretical and computational models of nanoparticle dynamics is given.