Magnetic helicity is a useful concept in the analysis of systems with extremely low resistivity, such as astrophysical systems. When resistivity is low, magnetic helicity is conserved over longer timescales, to a good approximation. Magnetic helicity dynamics are particularly important in analyzing solar flares and coronal mass ejections. Magnetic helicity is relevant in the dynamics of the solar wind. Its conservation is significant in dynamo processes, and it also plays a role in fusion research, such as reversed field pinch experiments.