This is a graduate-level introduction to quantitative concepts and methods in the science of living systems. It relies on a systems approach for understanding the physical principles operating in biology. Physical phenomena are treated at the appropriate spatio-temporal scale and phenomenological equations are used in order to reflect the system of interest. Biological details enter to the degree necessary for understanding specific processes, but in many cases the approach is not reductionist. This is in line with the approach taken by physics to many other complex systems. The book bridges the gap between graduate students' general physics courses and research papers published in professional journals. It gives students the foundations needed for independent research in biological physics and for working in collaborations aimed at quantitative biology and biomedical research. Also included are modern mathematical and theoretical physics methods, giving the student a broad knowledge of tools that can shed light on the sophisticated mechanisms brought forth by evolution in biological systems. The content covers many aspects that have been the focus of active research over the past twenty years, reflecting the authors' experience as leading researchers and teachers in this field.