Multiple slender structures subjected to a fluid flow are widely seen in engineering. Flow interference between the structures leads to a very high fluctuation of forces on the structures, structural vibrations, acoustic noise, or resonance, which in some cases can trigger failure, jeopardizing the residents'' lives. In engineering, fluid-induced forces, flow structures and vortex shedding frequencies are the major factors to consider in the design of structures. The knowledge of flow around two fluid-dynamically interfering cylinders is insightful for understanding the flow around more structures. This book encompasses the advancement of knowledge in physics of flow around two circular cylinders in a cross flow, including steady and fluctuating fluid forces, surface pressure, Strouhal numbers, flow structures, wake characteristics, flow switching mechanisms, possible interaction mechanisms, instabilities, and correlations between forces, reattachments, phase of vortex-shedding, Strouhal numbers. This book is intended for postgraduate students, researchers, engineers, and teachers in mechanical, civil, aeronautical, and ocean engineering.