Contemporary Japanese possesses three major types of words, (1) kango (Sino-Japanese words), (2) wago (Japanese native words), and (3) gairaigo (loanwords), and each word type is associated with three types of scripts, (1) kanji (Chinese characters), (2) hiragana, and (3) katakana. Kanji are a set of logographic/ideographic scripts, and both hiragana and katakana are phonetic syllabaries. Kanji are used for presenting kango and wago, while some wago are written only in hiragana. Katakana are used for presenting gairaigo. Moreover, katakana are unconventionally used to write kango and wago for the purposes of emphasis and so forth. This paper extensively examines katakana words including gairaigo as well as kango and wago written in katakana unconventionally. Many observers have commented that katakana words are increasing in Japanese writings. However, there is little empirical data to prove their increase in such writings. This dissertation pursues this question by hypothesizing that katakana words have been increasingly used in Japanese writings. Together with providing evidence of their increase, this dissertation captures differences in katakana word usage between different publication outlets, namely, magazines and newspapers as well as television commercials. In order to investigate these issues, a research project is conducted where vocabularies from the three types of media are collected on computer databases. The increase of katakana words in Japanese writings poses a problem to foreign learners of Japanese whose L1 is English. As reported in various research including Chikamatsu (1996), the learners generally experience difficulty in processing and comprehending katakana words. From the learners’ perspective, the increase of katakana words in Japanese writings means that the learners need to know more katakana words than ever before to read Japanese writings. Meanwhile, native readers of Japanese also express some difficulty with katakana words, as illustrated in the recent survey results by the National Institute for Japanese Language and the Japan Broadcasting Corporation. Both governmental agencies found that many Japanese had the experience of being unable to understand the meaning of some gairaigo. Their difficulty seems to partially be caused by the increase of gairaigo in Japanese writings. From the native readers’ perspective, the increase of gairaigo means that new gairaigo are continuously introduced in such writings, some of which are not deeply rooted in Japanese gairaigo inventory but have been introduced relatively recently, with the ensuing result that many people do not know the meaning of such words. Although it is clear that both foreign learners and native readers of Japanese have difficulty with katakana words due to the increase of such words, to date no linguistic analysis has been conducted to account for reasons of their difficulty. Thus, this dissertation examines such reasons, and then offers some suggestions as to how to make gairaigo more comprehensible from the point of view of script policy and pedagogical practice. And ultimately, some conclusions regarding the role of katakana in the Japanese orthographic system will be discussed in the light of the history of the Japanese language and its writing system.