Léon V. G. de la Vallée-Poussin was a Belgian mathematician born on March 14, 1869, and he is best known for his contributions to number theory and mathematical analysis. He made significant advancements in the study of prime numbers and was influential in the development of analytic number theory. In addition to his research, de la Vallée-Poussin was a dedicated educator, teaching at various universities. He is particularly noted for his work on the Prime Number Theorem, which describes the asymptotic distribution of prime numbers. His legacy continues to impact the field of mathematics today.