Lagrange's Theorem is a fundamental result in group theory, a branch of mathematics. It states that for any finite group, the order (number of elements) of any subgroup divides the order of the group. This means if you have a group with a certain number of elements, the sizes of its subgroups will always be factors of that number. The theorem is named after the mathematician Joseph-Louis Lagrange, who contributed significantly to the field of algebra. Lagrange's Theorem helps in understanding the structure of groups and is essential for further studies in abstract algebra and related areas.