Kantianism is a philosophical school based on the writings of the key German Idealist philosopher Immanuel Kant, and the philosophies that have arisen from the subsequent study of his writings. It was centered in Germany during the Age of Enlightenment of the late 18th and early 19th Century. To some extent it is synonymous with the German Idealism movement, although Kantianism also assumes acceptance of Kant's positions in Epistemology, Philosophy of Mind, Political Philosophy and (particularly) Ethics, in addition to his metaphysical Idealism. It is closely linked with the Romanticism movement.
In the 1780s and 1790s, Kant tried to refine Bishop George Berkeley's rather naive formulation of Idealism which, in general terms, is the theory that fundamental reality is made up of ideas or thoughts and not physical matter. At the same time, he attempted to bridge the two dominant philosophical schools of the 18th Century: Rationalism (which held that knowledge could be attained by reason alone, a priori), and Empiricism (which held that knowledge could be arrived at only through the senses, a posteriori). Kant's Transcendental Idealism claims that we know more than Berkeley's ideas in our minds, in that we also directly know of at least the possibility of "noumena" ("things-in-themselves"), which are both empirically and transcendentally real even if they cannot be directly and immediately known. The actual "phenomena" which we perceive and which we think we know are really just the way things appear to us and not necessarily real. See the section on the doctrine of Idealism for more details.
Kant's view of Ethics is deontological (i.e. it focuses on the rightness or wrongness of actions themselves, as opposed to the rightness or wrongness of the consequences of those actions or the character of the actor, and holds that ethical rules bind people to an ethical duty). It is founded on his view of rationality as the ultimate good, and his belief that all people are fundamentally rational beings. His major contribution was the theory of the Categorical Imperative which, at its simplest, states that one should act only in such a way that you would want your actions to become a universal law, applicable to everyone in a similar situation. See the section on the doctrine of Deontology for more details.
In the 1790s, there emerged in Germany the so-called "semi-Kantians", who altered features of Kant's system they viewed as inadequate, unclear or even wrong. These include Friedrich Schiller (1759 - 1805), Friedrich Bouterwek (1766 - 1828) and Jakob Friedrich Fries (1773 - 1843). The period from 1790 to 1835 was the age of the post-Kantian German Idealists, among whom Johann Gottlieb Fichte, Friedrich Schelling, Georg Wilhelm Friedrich Hegel and Arthur Schopenhauer were the most influential.
Schopenhauer's criticisms of the later German Idealists is seen by some as a sort of "back to Kant" movement, giving impetus to a Neo-Kantianism movement in the mid-19th and into the 20th Century, which yielded the Kantian analyses of such German philosophers as Kuno Fischer (1824 - 1907), Friedrich Lange (1828 - 1875), Hermann Cohen (1842 - 1918), Paul Natorp (1854 - 1924), Nicolai Hartmann (1882 - 1950), Ernst Cassirer (1874 - 1945), Wilhelm Windelband (1848 - 1915), Heinrich Rickert (1863 - 1936) and Ernst Troeltsch (1865 - 1923).