T.S. Eliot was influential both as a poet and as a literary critic during the first half of the twentieth-century. Much of his work is a seminal force in the modernist movement, and he was highly respected, even by those who didn't necessarily imbibe his literary technique or philosophy.

Modernist poetry and modernist artistic forms in general were partly a reaction against nineteenth-century Romanticism. The beginning of Eliot's "The Love Song of J. Alfred Prufrock" illustrates this as a kind of opening salvo against the past:

Let us go then, you and I,

When the evening is spread out against the sky,

Like a patient etherised upon a table.

With its allusion to medicine and cold science this jolting simile is, or is intended to be, a debunking of sentimentalized Romantic imagery. The entire tone of the poem is contrary to the displays of emotion typical of the Romantics and the Victorians. Prufrock is a little man afraid of everything, worrying himself over things like "eating a peach" and "wearing the bottoms of his trousers rolled." The language of the poem is deliberately stripped of adornment and often made to sound even pedestrian:

In the room the women come and go

Talking of Michelangelo.

Eliot's aesthetic is one of regret over a lost past and a sense that the modern age is cold and uninviting. In "The Wasteland" he transforms Chaucer's opening of The Canterbury Tales into a statement of pessimism:

April is the cruelest month....

The poem is a re-creation in a twentieth-century context of bits and pieces of the great literature of the past, always in such a way as to express discontent with the modern world and a sense of hopelessness:

A crowd flowed over London Bridge, so many,

I had not thought death had undone so many.

This is a paraphrase of verses from Dante's Inferno, the suggestion being that modern London is like hell and that its population is like the souls of the damned. In George Orwell's writings, both his early fiction and his essays, we can see an echo of Eliot's thought, despite the fact that Orwell is not usually one whom we would consider a central figure, specifically of the modernist movement.

Twentieth-century poets, from Ezra Pound to Ted Hughes, Sylvia Plath, and countless others, echoed the sobering, negative, and hopeless tone of Eliot's verse. His middle-period verse and his later poetry, such as "Ash Wednesday" and "The Four Quartets," are expressive of Eliot's "return" to religion, but also continue to be infused with a sense of resignation and pessimism:

Because I do not hope to turn again,

Because I do not hope,

Because I do not hope to turn.

Desiring this man's gift and that man's scope.

I no longer strive to strive towards such things

(Why should the aged eagle stretch its wings?)

Why should I mourn

The vanished power of the usual reign?

In both content and technique—the latter usually being either free verse or an alternation between free verse and more regular or conventional metrical forms—Eliot's work can be seen as an archetype of modernist poetry.

As a literary critic, Eliot was an iconoclast, expressing views about poetry and literature in general that in many ways were the antithesis of the standard views of the Romantic period. He disliked the poetry of Shelley, for instance, and his views on Shelley and other poets were repeated and amplified by critics who were extremely influential in the early twentieth-century academic world, such as F.R. Leavis. Though he later changed his views on Milton, an early essay on the author of Paradise Lost was a needed corrective to the nineteenth-century's idolization of Milton. Eliot's having been influenced by Donne was also instrumental in bringing about a revival of interest in Donne and the other Metaphysical poets. His essay on Hamlet is a criticism of Shakespeare's play, which, even those who have generally praised Eliot have found rather strange and eccentric, but it was influential in its time with some sectors of the literary community.

It is a testament to a writer's importance that the thinking of even those who may not have read either Eliot's poetry or his criticism has often been shaped by Eliot.

T. S. Eliot is considered one of the most important poets in the modernist movement. Many aspects of his poetry influenced subsequent forms of modernism in literature. His critical essays, especially "Tradition and the Individual Talent" were also influential.

One of the important innovations of Eliot's verse was borrowing the concept of free verse ("vers libre") from the French symbolists and implementing an equivalent in English. Unlike some late twentieth century free verse, Eliot's version maintains a ghost or scaffolding of traditional forms, so that the reader gets a sense of the writer interrogating the poetic tradition in which he is situated, both appropriating and rejecting it at the same time, in a gesture that follows his critical understanding of the way the modern poet interacts with his predecessors.

Another important modernist theme the Eliot develops is that of the fragment, as expressed in the ending of "The Wasteland":

Poi s'ascose nel foco che gli affina

Quando fiam uti chelidon - O swallow swallow

Le Prince d'Aquitaine à la tour abolie

These fragments I have shored against my ruins

Why then Ile fit you. Hieronymo's mad againe.

Datta. Dayadhvam. Damyata. 

What makes this ending so striking is that it contains four different languages, and material from five different previous works. The fragments are not explained or placed in coherent logical order, but starkly juxtaposed in a way that suggests the chaos into which modern society falls (for Eliot in part due to the decline of religion and monarchy) and how it leads to a "'dissociation of sensibility". 

Many poets found Eliot's success in writing this new style of verse liberating. The method of highly individuated references, including proper names, is also seen in Pound, the imagists, and many later poets, such as John Berryman. 

Eliot also worked for many years as a poetry editor at Faber & Faber, one of the leading publisher of poetry in England, and thus has an immediate personal influence on modern poetry, encouraging many younger poets and helping them develop in their craft.