To appear in the Encyclopedia of Astronomy and Astrophysics (Institute of Physics) December, 2000

Table of Contents

• INTRODUCTION
• NUCLEOSYNTHESIS IN THE EARLY UNIVERSE
• Weak Equilibrium and the ⁴He Abundance
• Deuterium - The Ideal Baryometer
• Helium-3 - Complicated Evolution
• Lithium-7 - The Lithium Valley
• FROM HERE AND NOW TO THERE AND THEN
• Deuterium
• Helium-4
• Lithium-7
• CONFRONTATION OF THEORY WITH DATA
• Comparison of BBN with Non-BBN Baryon Density Estimates
• COSMOLOGY CONSTRAINS PARTICLE PHYSICS
• SUMMARY
• REFERENCES