Taking as its basis a survey of the 20th century Korean lexicon, this paper explores its lexical properties through examination of its lexical character, its extralinguistic background and various lexical aspects, and provides an overview of important achievements in lexical studies through the construction and arrangement of lexical data and through the examination of lexical studies. The major results are as follows: First, three main properties of the 20th century Korean lexicon were found: (1) Although it consists of native words, Chinese words, and foreign words from the West, native words are conspicuous for the motivation process of forming words, and highly developed in terms of symbolic words and sense words. (2) Changes in politics and social structures in the 20th century are reflected both directly and indirectly in the Korean lexicon. (3) Complex aspects have appeared due to the expansion of the lexicon, the mass production of new words including foreign words, differentiation between North Korea and South Korea and between old and young generations, and the appearance of an Internet vocabulary in the lexicon of young generations. Second, three significant achievements in 20th century studies on the Korean lexicon were identified: (1) Following on the construction of a lexical database, standard words were established, dictionaries were written, frequencies of words were examined, and basic words were chosen. (2) The government and various civil organizations have focused on lexical purification, with satisfactory results. (3) Books on lexicology and lexical history were published, research on lexical fields and lexical relations was activated and methodologies for lexical education were explored. Lastly, the 20th century Korean lexicon evolved complex and diverse features in response to the demands of the times. On the one hand, lexical studies during this time showed great development and produced significant results, both in quantity and quality. On the other hand, problems continued to exist in areas such as: (ⅰ) limitations in awareness of the importance of the lexicon; (ⅱ) objectives, targets and methodologies of lexical studies; and (ⅲ) lack of research scholars in this field. These problems remain to be addressed by the 21st Korean lexicon.