Yeats's work can been seen as falling into three periods:

• Early period   in which he was able to reproduce the exotic, languid atmospheres of the Romentics.His use of Irish  folklore went together with an original and icreasingly complex symbolism, influenced by his reading of the French Symbolists and William Blake.
•  Middle period   the poet was influenced by Ezra Pound and so his style became more modern and he conceived his symbols as a way to evoke the universal myths and archetypes.
• Later period  covers the years of maturity, when he wrote A Vision in which he described his philosophy, and other collections of poems.
  

Symbols
Yeats frequently  alludes to events and characters in myth and history or  in the world around him; this includes mot only the more familiar territory of classical, Greek and Roman culture, but also Irish legend and History, esotericism and philosophy.
The ideas of symbol and image are central to the understanding of Yeats's poetry. They are not only devices he uses to present his themes, but they are also themes in themselves, in which truths are embodied in all their richness and complexity. To Yeats the symbol has a "visionary" dimension, it offers "revelation" since it has the power "to evoke indefinable and yet precise emotions".

Themes
Yeats's themes range from his faith in the beauty and eternity of art to the relationship between the poet and the Irish people and tradition.
The theme of the death becomes urgent; the poet distinguishes between animal and human experiences of death. The animal is unaware of death in advance and simply dies. Man, on the aother hand, dies many times before his death. Every defeat, such as unhappy experiences of love, but also every victory, are a series of deaths, and rebirths prefiguring the end of life. The central idea is that of preparing oneself with dignity for that instant.
The idea of heroic individual also held Yeats's  immagination. Lonelinessis a feature of his heroes because their superior qualities set them apart and distinguish them from the public men and the common mob.