The two reading systems
Every kanji has at least two types of readings. On'yomi (音読み — sound reading) are the Chinese-derived pronunciations imported when Japan adopted Chinese characters. Kun'yomi (訓読み — meaning reading) are the native Japanese pronunciations that already existed for the concept before writing was adopted.
For example: 山 has the on'yomi san/zan (as in 富士山, Fujisan) and the kun'yomi yama (as in 山道, yamamichi — mountain path). The same character, two completely different readings — and both are correct in different contexts.
When to use on'yomi versus kun'yomi
Compound words (jukugo) — two or more kanji together — almost always use on'yomi: 電話 (den-wa — telephone), 山岳 (san-gaku — mountains). Single kanji used as standalone words or verb stems almost always use kun'yomi: 山 (yama — mountain), 食べる (tabe-ru — to eat). These are not absolute rules — Japanese has exceptions — but they hold in the vast majority of cases.
Some kanji have no kun'yomi (the concept was imported from Chinese without a pre-existing Japanese word) and some have no on'yomi (they were created in Japan for purely native concepts). 萌 (moe) — the term for a particular aesthetic passion in anime culture — is an example of a kanji that gained a new kun'yomi reading in modern Japanese.
Special readings: jukujikun and ateji
Some kanji compound words have unique readings unrelated to either on'yomi or kun'yomi. These are called 熟字訓 (jukujikun). Examples: 今日 (kyō — today, not kon-nichi), 明日 (ashita — tomorrow, not mei-nichi), 大人 (otona — adult, not dai-jin). These special readings must simply be memorised — they are among the trickiest aspects of Japanese reading for learners.
See kanji in practice
Our reference pages show both readings for each character with examples.