American literary critic Harold Bloom describes him as "a superb craftsman, a lyric poet without rival, and surely one of the most advanced sceptical intellects ever to write a poem.

"[5] Shelley's reputation fluctuated during the 20th century, but since the 1960s he has achieved increasing critical acclaim for the sweeping momentum of his poetic imagery, his mastery of genres and verse forms, and the complex interplay of sceptical, idealist, and materialist ideas in his work.

[6][7] Among his best-known works are "Ozymandias" (1818), "Ode to the West Wind" (1819), "To a Skylark" (1820), "Adonais" (1821), the philosophical essay "The Necessity of Atheism" (1811), which his friend T. J. Hogg may have co-authored, and the political ballad "The Mask of Anarchy" (1819).

His other major works include the verse dramas The Cenci (1819), Prometheus Unbound (1820) and Hellas (1822), and the long narrative poems Alastor, or The Spirit of Solitude (1815), Julian and Maddalo (1819), and The Triumph of Life (1822).

[13][14] He was the eldest son of Sir Timothy Shelley, 2nd Baronet of Castle Goring (1753–1844), a Whig Member of Parliament for Horsham from 1790 to 1792 and for Shoreham between 1806 and 1812, and his wife, Elizabeth Pilfold (1763–1846), the daughter of a successful butcher.

[24] In his senior years, Shelley came under the influence of a part-time teacher, James Lind, who encouraged his interest in the occult and introduced him to liberal and radical authors.

[25] Prior to enrolling for University College, Oxford, in October 1810, Shelley completed Original Poetry by Victor and Cazire (written with his sister Elizabeth), the verse melodrama The Wandering Jew and the gothic novel St. Irvine; or, The Rosicrucian: A Romance (published 1811).

[30] In the winter of 1810–1811, Shelley published a series of anonymous political poems and tracts: Posthumous Fragments of Margaret Nicholson, The Necessity of Atheism (written in collaboration with Hogg) and A Poetical Essay on the Existing State of Things.

[35] Shelley's correspondence with Harriet intensified in July, while he was holidaying in Wales, and in response to her urgent pleas for his protection, he returned to London in early August.

[38] For a year from June 1811, Shelley was also involved in an intense platonic relationship with Elizabeth Hitchener, a 28-year-old unmarried schoolteacher of advanced views, with whom he was corresponding frequently.

Godwin, who had modified many of his earlier radical views, advised Shelley to reconcile with his father, become a scholar before he published anything else, and give up his avowed plans for political agitation in Ireland.

[44] Early in 1812, Shelley wrote, published and with Harriet personally distributed in Dublin three political tracts: An Address, to the Irish People; Proposals for an Association of Philanthropists; and Declaration of Rights.

The incident might have been real, a hallucination brought on by stress, or a hoax staged by Shelley in order to escape government surveillance, creditors and his entanglements in local politics.

Hearing nothing from Harriet in Switzerland, and unable to secure sufficient funds or suitable accommodation, the three travelled to Germany and Holland before returning to England on 13 September.

[69] A tour of Chamonix in the French Alps inspired "Mont Blanc", which has been described as an atheistic response to Coleridge's "Hymn before Sunrise in the Vale of Chamoni".

I met a traveller from an antique land, Who said—"Two vast and trunkless legs of stone Stand in the desart.... Near them, on the sand, Half sunk a shattered visage lies, whose frown, And wrinkled lip, and sneer of cold command, Tell that its sculptor well those passions read Which yet survive, stamped on these lifeless things, The hand that mocked them, and the heart that fed; And on the pedestal, these words appear: My name is Ozymandias, King of Kings; Look on my Works, ye Mighty, and despair!

[88] Shelley also published two political tracts under a pseudonym: A Proposal for putting Reform to the Vote throughout the Kingdom (March 1817) and An Address to the People on the Death of Princess Charlotte (November 1817).

[139] Shelley's badly decomposed body washed ashore at Viareggio ten days later and was identified by Trelawny from the clothing and a copy of Keats's Lamia in a jacket pocket.

[140] The day after the news of his death reached England, the Tory London newspaper The Courier printed: "Shelley, the writer of some infidel poetry, has been drowned; now he knows whether there is God or no.

[155] He advocated Catholic emancipation, republicanism, parliamentary reform, the extension of the franchise, freedom of speech and peaceful assembly, an end to aristocratic and clerical privilege, and a more equal distribution of income and wealth.

[159] Shelley's advocacy of nonviolent resistance was largely based on his reflections on the French Revolution and rise of Napoleon, and his belief that violent protest would increase the prospect of a military despotism.

"[162] In his later essay A Philosophical View of Reform, Shelley did concede that there were political circumstances in which force might be justified: "The last resort of resistance is undoubtably [sic] insurrection.

Reviewers often launched personal attacks on Shelley's private life and political, social and religious views, even when conceding that his poetry contained beautiful imagery and poetic expression.

[175] There was also criticism of Shelley's intelligibility and style, Hazlitt describing it as "a passionate dream, a straining after impossibilities, a record of fond conjectures, a confused embodying of vague abstraction".

[7] Shelley was a major influence on a number of important poets in the following decades, including Robert Browning, Algernon Swinburne, Thomas Hardy and William Butler Yeats.

[5] Shelley-like characters frequently appeared in nineteenth-century literature; they include Scythrop in Thomas Love Peacock's Nightmare Abbey,[179] Ladislaw in George Eliot's Middlemarch and Angel Clare in Hardy's Tess of the d'Urbervilles.

[180] Twentieth-century critics such as Eliot, Leavis, Allen Tate and Auden variously criticised Shelley's poetry for deficiencies in style, "repellent" ideas, and immaturity of intellect and sensibility.

[182] American literary critic Harold Bloom describes him as "a superb craftsman, a lyric poet without rival, and surely one of the most advanced sceptical intellects ever to write a poem".

Among the most notable of these are:[185][186] Shelley's long-lost "Poetical Essay on the Existing State of Things" (1811) was rediscovered in 2006 and subsequently made available online by the Bodleian Library in Oxford.

[189] Fiona Sampson has said: "In recent years Percy's corrections, visible in the Frankenstein notebooks held at the Bodleian Library in Oxford, have been seized on as evidence that he must have at least co-authored the novel.