Henri Léon Lebesgue was a French mathematician born on June 28, 1875, and is best known for his contributions to the field of measure theory. His work laid the foundation for modern analysis and provided a rigorous way to define the concept of integration, which is crucial in calculus. The Lebesgue integral, named after him, extends the idea of integration beyond traditional methods. Lebesgue's influence also reached areas such as real analysis and topology. He published several important papers and books, including "Leçons sur l'intégration et la recherche des fonctions primitives." His innovative ideas have had a lasting impact on mathematics, making him a key figure in the development of 20th-century mathematical thought.