Languages are in the state of constant changes; English is not exceptional. Changes of every sort in the present-day English have taken place simultaneously in different aspects of a language namely phonetics

morphology

syntax and vocabulary. Vocabulary

as the most active element and the mirror of society and culture

is changing most radically and constantly. This thesis attempts to explore basic laws and causes behind lexical changes in contemporary English by analyzing and expounding concrete changes since the 20th century in lexical field.Beginning with the general introduction and the history of English vocabulary development

the thesis first illustrates three basic laws behind lexical changes in contemporary English: (1) adaptation law: lexical changes are closely based on and timely adapted to the developments of the modern society; (2) gradualistic law: lexical changes are constant and gradual

and do not happen in a sudden flight; (3) limited novelty law: most of the lexical changes are built on old forms or meanings of existing vocabulary

while totally creative new words are rare. Then

the author studies causes of lexical changes: (1) expressions for the new things; (2) the desire for novelty; that is why lexical changes have been happening constantly in contemporary English. At last

a complete summary is given and the author looks forward to the future of English development.