Hegelianism is a philosophical school based on the writings of the German Idealist philosopher Georg Wilhelm Friedrich Hegel, and the philosophical tradition that began with him. It was centered in Germany during the mid-19th Century.
Hegel's major works include "The Phenomenology of Spirit" (1807), "Encyclopaedia of the Philosophical Sciences" (1817) and "Philosophy of Right" (1821). His works are considered notoriously difficult to understand, but his philosophy can perhaps be summed up by the motto "the rational alone is real".
He advocated a kind of historically-minded Absolute Idealism, in which the universe would realize its spiritual potential through the development of human society, and in which mind and nature can be seen as two abstractions of one indivisible whole Spirit. Hegel developed his theory out of the Subjective Idealism (or Transcendental Idealism) of Immanuel Kant.
Hegel was also probably the first philosopher to think of history itself as a dialectical process, in which reality can be understood through a three-stage dialectic, starting with the indeterminate concept (or thesis) to the determinate concept (or antithesis) and then to the resolution (or synthesis). Hegel saw "Geist" (the absolute mind or spirit) developing through history, with each period having a Zeitgeist (spirit of the age). Hegel's theory of the dialectic was the inspiration for the Dialectical Materialism of Karl Marx and Marxism.
Hegel's immediate followers in Germany are generally divided into the Hegelian Rightists (also known as Right Hegelians or Old Hegelians) and the Hegelian Leftists (also known as Left Hegelians or Young Hegelians).
The Rightists (following the lead of some of the other German Idealists) developed Hegel's philosophy along lines which they considered to be in accordance with Christian theology, and took his philosophy in a politically and religiously conservative direction. The Right Hegelians felt that the series of historical dialectics had been completed, and that Prussian society as it existed was the culmination of all social development to date. They included Johann Philipp Gabler (1753 - 1826), Karl Friedrich Göschel (1784 - 1861), Johann Karl Friedrich Rosenkranz (1805 - 1879) and Johann Eduard Erdmann (1805 - 1892).
The Leftists accentuated the anti-Christian tendencies of Hegel's system, and believed that there were still further dialectical changes to come, and that the Prussian society of the time was far from perfect. Many of the more radical Young Hegelians disagreed with many of Hegel's conclusions, but they found his dialectical approach to be very useful. They included Ludwig Feuerbach (1804 - 1872), David Friedrich Strauss (1808 - 1874), Johann Paul Friedrich Richter (1763 - 1825), Bruno Bauer (1809 - 1882), Friedrich Engels (1820 - 1895) and Karl Marx (who eventually fell out with the other Young Hegelians, but nevertheless went on to develop his theory of Dialectical Materialism from Hegel's principles). Max Stirner (1806 - 1856) socialized with the Left Hegelians but built his own philosophical system, largely opposing that of these thinkers, and was influential in the development of Nihilism, Existentialism and Anarchism.
The British Idealism movement of the mid-19th Century to the early 20th Century revived interest in the works of Kant and Hegel. The leading figures in the movement were T. H. Green (1836 - 1882), F. H. Bradley (1846 - 1924), Bernard Bosanquet (1848 - 1923), J. M. E. McTaggart (1866 - 1925), H. H. Joachim (1868 - 1938) and J. H. Muirhead (1855 - 1940).
There were also Hegelian philosophers in Denmark, Poland, France, North America and Italy, where Hegelianism inspired the "Actual Idealism" and Fascism of Giovanni Gentile (1875 - 1944).