I (И и; italics: И и) is a letter used in almost all ancient and modern Cyrillic alphabets.

It commonly represents the close front unrounded vowel /i/, like the pronunciation of ⟨i⟩ in "machine", or the near-close near-front unrounded vowel /ɪ/, like the pronunciation of ⟨i⟩ in "bin".

The Cyrillic letter I was derived from the Greek letter Eta (Η η). This is why the earliest (up to the 13th century) shape of Cyrillic ⟨И⟩ was ⟨H⟩.

The name of the Cyrillic letter I in the Early Cyrillic alphabet was ижє (iže), meaning "which".

In the Cyrillic numeral system, the Cyrillic letter I had a value of 8.

In the Early Cyrillic alphabet there was little or no distinction between the letter ⟨И⟩ and the letter ⟨І⟩ which was derived from the Greek letter Iota (Ι ι). They both remained in the alphabetical repertoire because they represented different numbers in the Cyrillic numeral system, eight and ten, and are therefore sometimes referred to as octal I and decimal I. Today they co-exist in Church Slavonic (with no pronunciation difference) and in Ukrainian (representing actual pronunciation differences). Other modern orthographies for Slavic languages eliminated one of the two letters during alphabet reforms of the 19th or 20th centuries: Russian, Macedonian, Serbian and Bulgarian languages use only ⟨И⟩, whereas Belarusian uses only ⟨І⟩.

The Cyrillic script ( /sɨˈrɪlɪk/) or azbuka is an alphabetic writing system developed in the First Bulgarian Empire during the 10th century AD at the Preslav Literary School. It is the basis of alphabets used in various languages, past and present, in Eastern Europe and Asia, especially those of Slavic origin, and non-Slavic languages influenced by Russian. As of 2011 around 252 million people in Europe and Asia use it as official alphabet for their national languages. About half of them are in Russia.

Cyrillic is derived from the Greek uncial script, augmented by ligatures and consonants from the older Glagolitic alphabet and Old Bulgarian for sounds not found in Ancient Greek. It is named in honor of the two Eastern Roman Empire brothers, Saints Cyril and Methodius, who created the Glagolitic alphabet earlier on. Modern scholars believe that Cyrillic was developed and formalized by early disciples of Cyril and Methodius (such as Clement of Ohrid).

With the accession of Bulgaria to the European Union on 1 January 2007, Cyrillic became the third official script of the European Union, following the Latin and Greek scripts.