字母频率，指的是各个字母在文本材料中出现的频率。常被应用于密码学，尤其是可破解古典密码的频率分析。在英语中最常见的字母是e。而在铅字印刷时代，人们已根据经验在Linotype排字机上将字母按常用与否排列成etaoin shrdlu cmfwyp vbgkjq xz 。还有，摩斯电码中越常用的字母，其编码符号就越短；而发出各字母的用时由快到慢顺序是e it san hurdm wgvlfbk opjxcz yq。数据压缩技术中也有相似的方法，如霍夫曼编码就是按来源符号出现的机率大小去编码。

The frequency of letters in text has been studied for use in cryptanalysis, and frequency analysis in particular, dating back to the Iraqi mathematician Al-Kindi (c. 801–873 CE), who formally developed the method (the ciphers breakable by this technique go back at least to the Caesar cipher invented by Julius Caesar, so this method could have been explored in classical times).