Recent breakthroughs in MRI methodology and biophysical modelling of the MR signal brought quantitative in vivo MRI markers with sub-millimeter resolution and high specificity to microscopic tissue compartments within reach (e.g. MRI markers for myelin, axon, or iron concentration). However, ex vivo histology remains the gold standard against which these MRI markers have to be validated before they can be reliably used for clinical research or studying neuroscientific questions. Today, several challenges need to be overcome to achieve quantitative validation of these MRI markers: (i) understanding and modelling the changes occurring post mortem, e.g. autolysis, temperature changes and fixation, which significantly alter the MRI signal and the tissue morphology, (ii) accounting for the scale gap between histological methods, which is typically performed on small 2D sections of tissue (millimeters to few centimeters) with a microscopic resolution (~ 1 micron), and macroscopic MRI, which is performed on the whole three-dimensional brain at a macroscopic resolution (~ 1 millimeter), (iii) finding the most reliable, reproducible, and quantitative histological techniques to serve as the gold standard measurement tools for automated quantification of tissue compartments, ideally over the entire brain. This symposium is comprised of four lectures that introduce a broad range of promising methods to tackle these challenges.