Changes of iron and myelin content in the brain can indirectly indicate alterations in brain function and are often associated with various neurodegenerative diseases. The processes of iron deposition and myelination/demyelination are known to occur during brain development and aging, and are often accompanied by changes in susceptibility of brain tissue. One method for quantifying such changes is Quantitative Susceptibility Mapping (QSM), which is a magnetic resonance imaging technique based on Gradient-Recalled Echo (GRE) sequences to measure tissue susceptibility. QSM can be used to quantify the deposition of iron in the gray matter and myelination/demyelination in the white matter. The trajectory of iron and myelin during brain development can be modeled using exponential and poisson functions based on QSM images, which can yield results consistent with other quantitative tools such as histological quantification. Based on this methodology, researchers have constructed a longitudinal atlas from QSM images, which provides a basis for exploring the role of iron and myelin in brain aging and neurodegenerative diseases. In this review, we first introduce the principle of QSM and its applications to the studies of brain development and aging.