In modern Japanese, katakana are most often used for transcription of words from foreign languages (gairaigo). For example, "television" is written ''terebi'' (テレビ|). Similarly, katakana is usually used for country names and foreign place and personal names. For example America is written アメリカ ''Amerika'' and John is written ジョーン (Jōn).
Katakana are also used for onomatopoeia, letters used to represent sounds, for example ''hii'' (ヒー|), meaning "sigh" would usually be written in katakana.
Technical and scientific terms, such as the names of animal and plant species and minerals are also commonly written in katakana.
Katakana are also often used for transcription of Japanese company names. For example Suzuki is written スズキ,and Mitsubishi is written ミツビシ, Katakana are also used for emphasis, especially on signs, advertisements, and hoardings. For example, it is common to see ゴミ ''gomi'' (trash) or メガネ ''megane'' (glasses), and words to be emphasized in a sentence are also sometimes written in katakana.
Pre-world war 2 official documents mix katakana and kanji in the same way that hiragana and kanji are mixed in modern Japanese texts, that is, katakana were used for okurigana and particles such as ''wa'' or ''o''.
Telegrams are written in katakana, and before the introduction of multibyte characters in computer systems in the 1980s, computer output was entirely in katakana.
Although words borrowed from ancient Chinese are usually written in kanji, loanwords from modern Chinese which are borrowed directly rather than using the Sino-Japanese on'yomi readings, are often written in katakana. Examples include
| Katakana | Kanji | Rōmaji | Meaning |
|---|---|---|---|
| マージャン | 麻雀 | mājan | mahjong |
| ウーロン | 烏龍茶 | ūroncha | oolong tea |
| チャーハン | 炒飯 | chāhan | fried rice |
| チャーシュー | 叉焼 | chāshū | roast pork |
| シューマイ | 焼売 | shūmai | a kind of dim sum |
| ラーメン | 拉麺 | rāmen | Chinese-style noodles |
Katakana are sometimes used instead of hiragana as furigana to give the pronunciation of a word written in roman characters, or for a foreign word which is written as kanji for the meaning, but intended to be pronounced as the original.
Katakana are also sometimes used to indicate words being spoken in a foreign or otherwise unusual accent, by foreign characters, robots etc. For example, in a manga, the speech of a foreign character or a robot may be represented by コンニチワ (''konnichiwa'') instead of the more usual hiragana こんにちは (''konnichi wa'').
Katakana are also used to indicate the on'yomi readings of a kanji in a kanji dictionary.
Some personal names are written in katakana. This was more common in the past, hence elderly women often have katakana names.
It is very common to write words with difficult-to-read kanji in katakana. This phenomenon is often seen with medical terminology. For example, in the word "dermatologist", 皮膚科, ''hifuka'', the second kanji, 膚, is considered difficult, and thus the word ''hifuka'' is commonly written as 皮フ科 or ヒフ科 in katakana. Similarly, difficult kanji such as 癌 ''gan'', "cancer", are often written in katakana.