Hegel's first and most important major work is the Phenomenology of Spirit (or Phenomenology of Mind). During his life he also published the Encyclopedia of the Philosophical Sciences, the Science of Logic and the (Elements of the) Philosophy of Right. A number of other works on the philosophy of history, religion, aesthetics, and the history of philosophy were compiled from the lecture notes of his students and published posthumously.
Hegel's works have a reputation for their difficulty, and for the breadth of the topics they attempt to cover. Hegel introduced a system for understanding the history of philosophy and the world itself, often called a "dialectic": a progression in which each successive movement emerges as a solution to the contradictions inherent in the preceding movement. For example, the French Revolution for Hegel constitutes the introduction of real freedom into western societies for the first time in recorded history. But precisely because of its absolute novelty, it is also absolutely radical: on the one hand the upsurge of violence required to carry out the revolution cannot cease to be itself, while on the other, it has already consumed its opponent. The revolution therefore has nowhere to turn but on to its own result: the hard-won freedom is consumed by a brutal Reign of Terror. History, however, progresses by learning from its mistakes: only after and precisely because of this experience can one posit the existence of a constitutional state of free citizens, embodying both the (allegedly) benevolent organizing power of rational government and the revolutionary ideals of freedom and equality.
In contemporary accounts of Hegelianism — to undergraduate classes, for example — Hegel's dialectic often appears broken up for convenience into three moments called "thesis" (in our example, the revolution), "antithesis" (the terror which followed), and "synthesis" (the constitutional state of free citizens). Hegel did not use this classification at all himself, though: it was developed earlier by Fichte in his loosely analogous account of the relation between the individual subject and the world. Serious Hegel scholarship does not recognize the usefulness of this classification for shedding light on Hegel's thought, so its employment by a writer in a discussion of Hegel is a good indicator that that writer has very little familiarity with or understanding of Hegel's philosophy.
Hegel used this system to explain the whole of the history of philosophy, science, art, politics and religion, but many modern critics point out that Hegel often seems to gloss over the realities of history in order to fit it into his dialectical mold. Karl Popper, a critic of Hegel in The Open Society and Its Enemies, suggests that the Hegel's system forms a thinly veiled justification for the rule of Frederick William III, and that Hegel's idea of the ultimate goal of history is to reach a state approximating that of 1830s Prussia. This view of Hegel as an apologist of state power and precursor of 20th century totalitarianism was criticized thoroughly by Herbert Marcuse in his Reason and Revolution: Hegel and the Rise of Social Theory, on the grounds that Hegel was not an apologist for any state or form of authority simply because it existed: for Hegel the state must always be rational. Arthur Schopenhauer despised Hegel on account of the latter's historicism (among other reasons), and decried Hegel's work as obscurantist "pseudo-philosophy". Many other newer philosophers who prefer to follow the tradition of British Philosophy and Kantianism have made similar statements.
In the 20th century, Hegel's philosophy underwent a major renaissance. This was due partly to the rediscovery and reevaluation of him as the philosophical progenitor of Marxism by philosophically oriented Marxists, partly through a resurgence of the historical perspective that Hegel brought to everything, and partly through increasing recognition of the importance of his dialectical method. Some figures associated with this renaissance are Herbert Marcuse, Theodor Adorno, Ernst Bloch, Alexandre Kojeve and Gotthard Günther. The Hegel renaissance also highlighted the significance of Hegel's early works, i.e. those published prior to the Phenomenology of Spirit. More recently two prominent American philosophers, John McDowell and Robert Brandom (sometimes, half-seriously, referred to as the Pittsburgh Hegelians), have exhibited a marked Hegelian influence.