In Spanish and Catalan "Ü" is the letter "U" with a diaeresis, indicating that "U" is pronounced in a position where it would not normally be, specifically between a "G" and an "E" or "I" in Spanish and Catalan and between a "Q" and "E" or "I" in Catalan
Since in Spanish the pronunciation of "G" changes from /g/ to /x/ (same as "J") when followed by "I" or "E" (vocales débiles, weak vowels), a silent "U" can be added after it to restore the /g/ sound. For example, guerra (war) is pronounced /geRa/, not /xeRa/. If the "U" is to be pronounced, then a diaeresis is added; for example, lingüistica (linguistics) is pronounced /lingwistika/, not /lingistika/. Similarly, in Catalan.
Another use exclusive of Catalan language consist into separate two vowels that without the diaeresis would sound together in the same syllable. For example diurn would sound /djwrn/ but diürn sounds /di'urn/ (two syllables).
In the Chinese romanizations pinyin, Wade-Giles, and the German-based Lessing-Othmer, "Ü" is used to represent the vowel of 玉 (jade) and 雨 (rain). Pinyin uses "Ü" only when ambiguity could arise, while Wade-Giles and Lessing use it in all situations.