Bulges lie at the center of spiral galaxies. Until recently, they were thought to host uniquely old stellar populations and thus provide a key for understanding galaxy formation. Recent observations from the ground and space have drastically changed our view on the nature of bulges and shown that they can also contain dust, gas, and star-forming regions. This timely volume presents review articles by a panel of international experts who gathered at a conference at the Space Telescope Science Institute, Baltimore, to address several fundamental questions: What is a bulge? When and how did bulges form? And, on what timescales? This volume provides a state-of-the-art picture of our new understanding of these fundamental building-blocks of galaxies, and a stimulating reference point for all those interested in galaxy formation.