In nine new essays, distinguished philosophers of science take on outstanding philosophical issues that arise in the exploration of the foundations of contemporary, especially physical scientific theories. In the first part of the book issues of scientific method are explored. What are we asking when we pose scientific "why?" questions? How does probability play a role in answering such questions? What are scientific laws of nature? How can we understand what abstract theories are telling us about the world? What is the structure of the theories we use to explain the observable phenomena? Finally, how do theories evolve over time and what consequence do such changes have for our intuition that science is seeking the truth? In the second part of the volume, foundational issues are explored in a number of crucial physical theories. What do our best available theories tell us about space and time? When we apply quantum theory to fields or other systems with infinite degrees of freedom, what new foundational puzzles appear and how might a theory of interpretation deal with them? Finally, what are the crucial foundational issues in statistical mechanics, where probabilities are applied to explain macroscopic thermal phenomena?