The physical foundations of nanoparticle magnetism are discussed in this chapter. Critical factors, such as nanoparticle chemistry, nanoparticle size and the impact of static and dynamic magnetic fields on nanoparticles are discussed. Of particular importance for biological/medicinal applications is the ability of magnetic nanoparticles to heat their surrounding medium when subjected to an external AC field. Another promising future application of magnetic nanoparticles is their mechanical rotation in external rotating magnetic fields. This chapter ends with the discussion of (ultra)sound generation by magnetic nanoparticles, which has recently emerged as a novel modality for triggered drug delivery and increased tissue penetration.