The category of foundational quantum physics encompasses the set of physical phenomena, experiments and theories from the late 19th and early 20th century that led to the development of quantum mechanics during the early 20th century. These effects were important as they clearly indicated that the then currently thriving classical theories of physics were insufficient to explain the phenomena being witnessed.
Much of the foundational work led to what we call today atomic physics, the theory of atoms as described by quantum mechanics. By the late 1930s and early 1940s, the foundational period was over, as the theory was sufficiently evolved and rich to lead to more modern work in nuclear physics, particle physics and quantum field theory.