This volume contains original contributions made at the sixth session of the Minkowski Institute international conference "Nature and Ontology of Spacetime." It continues the ambition of debating open ontological and epistemological problems in the philosophy and physics of spacetime. In this volume, topics covered include the reality of Minkowski spacetime ontology, the nature of time, quantum gravitation, inertia and relativistic phenomena, as well as aspects of the mathematical structure of the special and general theory of relativity. The volume will be of interest to students in philosophy of science, researchers in spacetime physics, and to any reader who wants to study contemporary issues and debates in modern philosophy and physics.