It is very well known that asphalt mixtures rutting is one of the major distress in flexible pavements, but what is it and why does it happen? What are the main parameters affecting the behavior of asphalt mixture? This book mainly shows the effect of aggregate gradation on the permanent deformation in asphalt mixtures. Additionally, the packing theory is also discussed. The outcomes of this book are useful to assess flexible pavements material before construction and for existing pavements.