Space Weather is influenced by phenomena operating across a multitude of scales, from the large-scale expulsion and evolution of coronal mass ejections (CMEs) from the solar corona, to particle-scale interactions within the Earth’s radiation belts. My interests within plasma simulations focus on both large-scale fluid physics and kinetic processes and the interplay between them. In this seminar I will start with Sun-to-Earth studies of CMEs and review some of the largest geomagnetic storms on record and examine under what conditions “Carrington-scale” events are possible. I will discuss how these impulsive injections of energy flow through the coupled Sun-Earth system into the Earth’s magnetosphere and how magnetohydrodynamic (MHD) modelling forms the backbone of current space weather forecasting efforts. I will then move on and examine how hazardous non-thermal particle distributions, not-considered within ideal MHD, can be rapidly created within the magnetosphere, and how these can feedback to influence the large-scale MHD dynamics and thus pose an outstanding challenge to space weather forecasting.