How can we learn about dynamic fine structures that are far too small to be resolved with the light microscope without destroying the active living cell? Examples spanning the last half century show how polarized light microscopy can-and should-continue to provide an attractive window for such studies. Long before microtubules were found with electron microscopy, or their assembly properties were biochemically characterized in isolated cell-free systems, the dynamic fine structure of the mitotic spindle and assembly properties of its microtubules were revealed in living cells by polarized light microscopy. More recently, the polarizing microscope was improved, by invention of the new Pol-Scope, so that quantitative measurements of bire-fringence retardation and axes could be made rapidly for all image pixels independent of their birefringence axis orientation. In addition, the centrifuge polarizing microscope, just developed, allows us to follow the dynamic ordering of fine structures in living cells as they become stratified or restructured by centrifugal acceleration of up to ten thousand times gravity. The significance of these technological advances is discussed-Inoué, S. Windows to dynamic fine structures, then and now.