This book is an introduction to the basic concepts that underlie the process ofphotosynthesis,as well as a description of the current understanding of the subject.Because photosynthesis is such a complex process that requires some knowledge of many different fields ofscience to appreciate,it can be intimidating for a person who is not already conversant with the basics of all these fields.For this reason,a brief overview is provided in the first chapter,introducing and summarizing the main concepts.This chapter is then followed by a more in-depth treatment ofeach ofthe main themes in later chapters. Photosynthesis is perhaps the best possible example ofa scientific field that is intrinsically interdisciplinary.Our discussion of photosynthesis will span time scales from the cosmic to the unimaginably fast,from the origin ofthe Earth 4.5 billion years ago to molecular processes that take less than a picosecond.This is a range ofnearly thirty orders of magnitude.In order to appreciate this extraordinary scale,we will need to learn a range ofvocabularies and concepts that stretch from geology through physics and chemistry,to biochemistry,cell and molecular biology,and finally to evolutionary biology.Any person who wishes to appreciate the big picture ofhow photosynthesis works and how it fits into the broad scope of scientific inquiry needs to have at least a rudimentary understanding of all these fields of science.This is an increasingly difficult task in this age of scientific specialization,because no one can truly be an expert in all areas.This book attempts to provide the starting point for a broadly based understanding of photosynthesis,incorporating key concepts from across the scientific spectrum.The emphasis throughout the book will be on molecular-scale mechanistic processes. Many of the concepts that we will explore in this book require an understanding of basic concepts ofphysical chemistry,including thermodynamics,kinetics and quantum mechanics.It is beyond the scope ofour broad,and therefore necessarily brief,treatment of photosynthesis to provide a comprehensive background in these areas that form the core of the mechanistic aspects of the subject.However,some modest understanding of these physical principles is essential to be able to appreciate the essence ofthe photosynthetic process. This need is addressed in the Appendix, which introduces the physical basis of light and energy.This appendix can either be read as a preface to the book as a whole or consulted as needed as a reference. The book is aimed at advanced undergraduate and beginning graduate students in a range of disciplines,including life sciences,chemistry and physics.An understanding of basic principles ofchemistry,physics and biology is assumed.