Biology is the study of life. The first organisms appeared on the planet over 3 billion years ago and, through reproduction and natural selection, have given rise to the 8 million or so different species alive today. Estimates vary, but over the course of evolution, 4 billion species could have been produced. Most of these flourished for a period of time, and then became extinct as new, better adapted, species took their place. This diversity makes biology both an endless source of fascination and a considerable challenge. German naturalist Gottfried Reinhold coined the word “biology” in 1802, but our understanding of living organisms only started to grow rapidly with the advent of techniques and technologies developed in the 18th and 19th centuries. Not least, the invention of the microscope and the realisation that natural selection is the process that has driven the evolution of life. Biologists attempt to understand the living world at all levels using many different approaches and techniques. At one end of the scale is the cell, its molecular construction and complex metabolic reactions. At the other end of the scale, biologists investigate the interactions that make whole ecosystems function. Many areas of research in biology are extremely challenging, and many discoveries remain to be made. Biology is still a young science and great progress is expected in the 21st century. This progress is sorely needed at a time when the growing human population is placing ever-greater pressure on food supplies and on the habitats of other species, and is threatening the very planet we occupy.