This encyclopaedia presents biographical entries on internationally renowned scientists-biologists, chemists, physicists and mathematicians, who have made important contribution to science. These scientists have been selected from the earliest times to the 20thcentury. This also covers a selection of people from philosophy, engineering and technology. The entries contain biographical data, place and date of birth, posts held etc. but do not give exhaustive personal details about the subjects family, prizes, honorary degrees, etc. However, a sincere effort has been made to include their main scientific achievements and the nature and importance of these achievements. Further, an attempt is made to include people who have produced major advances in theory or have made influential or well known discoveries. Certain entries have been quite lengthy because we have found them interesting. It is hoped that the reader will find the entries useful. This encyclopaedia will be of immense value to the students of biology, chemistry, mathematics, and physics and to research scientists and engineers working on science projects.