In this book, We formulate a model to describe the relaxation of single domain magnetic nanoparticles to incorporate the effect of dipolar interaction as well as particle size distribution on the measured relaxation spectra and irreversible, nonequilibrium magnetization response in field-cooled (FC) and zero-field-cooled (ZFC) cases. Further, we analyze the slow dynamics and concomitant memory (aging) effects seen in nanomagnetic systems on the basis of two separate paradigms : superparamagnets and spinglasses. Then, we study the long-time behaviour of an underdamped Brownian particle moving through a viscous medium in a periodic potential in the presence of an externally applied space-dependent highly oscillating force. The effect of dissipation and the essential role of the boundary is clarified in the context of dissipative diamagnetism. Finally, we present an overview of coherence-to-decoherence transition in the realm of dissipative diamagnetism at zero temperature.