The year 2015 was designated by the United Nations General Assembly as the Year of Light and Light-based Technologies, and also marks the anniversaries of a number of significant historical events related to light. In 1015, Ibn Al-Haytham published his book of optics; in 1815, Fresnel first proposed the notion that light is actually a wave; James Clerk Maxwell then firmly established this concept with his electromagnetic theory of light propagation; and Einstein announced his discovery of the photoelectric effect, demonstrating that light is made of photons in 1905, followed in 1915 by his general theory of relativity, in which light plays a central role. This book presents lectures from the International School of Physics Enrico Fermi summer school: Frontiers in Modern Optics, held in Varenna, Italy, in June and July 2014. The school attempted to give a broad and modern overview of the field of optics in a series of lectures addressing ongoing topics of research. Subject areas include: nonlinear optics; light as an investigation tool in modern physics; and detection and imaging. A unique feature of the book is that each chapter has been prepared as a collaborative effort between students at the school and lecturers. This approach has proved very successful and may well provide a model for the future.