R. Asher b. Jehiel (also known as Asheri and Rosh; c. 1250–1327), was first taught by his father, one of the Hasidei Ashkenaz, who was a follower of R. Judah b. Samuel he-Hasid, and his elder brother. He spent some time in France, apparently in Troyes and then lived in Cologne and Coblenz. From there he moved to Worms, where his teacher R. Meir b. Baruch of Rothenburg had been appointed rabbi in 1281. R. Meir esteemed his pupil, and appointed him a member of the local bet-din. After the imprisonment of R. Meir, R. Asher became the acknowledged leader of German Jewry and headed the unsuccessful efforts to obtain his master's release, toward which he was prepared to contribute a considerable portion of his assets. He distinguished himself for his activities during the period of the Rindfleisch massacres (1298), and for his decisions on matters arising from the resulting disruption of family and communal life. Fearing a similar fate to that of R. Meir of Rothenburg, R. Asher left Germany in 1303. The following year, he reached Barcelona, via north Italy and Provence, where he was welcomed with great honor by R. Solomon b. Abraham Adret. In 1305 he accepted the position of rabbi in Toledo. His son, R. Judah, relates that shortly thereafter, R. Asher turned down a request of the German authorities that he return to his native country, for which they were prepared to provide an imperial letter of safe-conduct and an escort of 50 soldiers.

He opposed customs which had been influenced by the Christian environment, such as, granting equal rights of inheritance to husband and wife and bequeathing the whole estate to the oldest son, as was the custom among the nobility; chaining of debtors; and compelling a husband to grant his wife a divorce on her declaration of her unwillingness to live with him. His vast influence and moral stature enabled him to overcome the difficulties which he encountered in those activities, and his spiritual influence was acknowledged even by the Castilian queen, Maria de Molina. His responsa sometimes reflect the modesty and humility that typified the German school, and at others, the firmness and authority of one speaking in the name of the supreme political and judicial body of Spanish Jewry. When the rabbi of Valencia insisted on his view in defiance of accepted practice and the opinion of R. Asher, the latter threatened him with capital punishment, if all the other deterrents enumerated in a letter to one of the scholars of the community should prove of no avail (Responsa, 107, 6). Despite his reservations and doubts as to the right of the rabbis to impose capital punishment, he nonetheless permitted them to act according to the custom prevalent in Spain, and consented to sentences of mutilation, particularly in the case of informers. R. Asher introduced into Spain the system of study of the tosafists and tried to establish a German minhag. He is regarded as one of the outstanding halakhic authorities who put the final seal to the work of the German and French codifiers, joining to it the Spanish halakhah. True to the methods of the tosafists, he subjected the statements of the rishonim and geonim to a critical examination and did not hesitate to disagree with them whenever talmudic sources did not support their view and conclusions. Virtually all the communities of Spain referred their problems to him and students flocked to his yeshivah from all Europe, including Russia. When he encountered matters not specifically prohibited in the Torah, R. Asher was prepared to abandon his own opinion in the face of strong opposition, particularly for the sake of peace, but he never hesitated from taking a strong stand against undesirable developments in the communal life. In answer to a complaint that members of distinguished families had not been appointed as cantors, he stated forcibly that neither distinguished descent, nor the possession of a pleasant voice should be the criterion, but only moral standing (ibid., 4:22).

His negative attitude toward philosophy did not extend to science generally and he encouraged Isaac b. Joseph Israeli to write his Yesod Olam. He was familiar with German law and Spanish common law but his knowledge of Arabic was limited to the spoken language. Having lost all his property in Germany, he lived under conditions of financial stress and his son notes that his father's assets at the time of his death were insufficient for the execution of his will. R. Asher nevertheless continued in Spain his ancestral custom of tithing all his income.

ספירת-דפים משובשת. דף [3,א-5,א]: הקדמה ... אשר דיבר הרב ... ר' אלישע... דף [5]: וזהו דבר השמטה מספר לחם שמים שחיבר ... מהר"י [יעקב] נר"ו אב"ד מעמדן במסכת ר"ה פ"ד מ"ז. דף [6]: הקדמה [שניה]. דף [7,ב-8,א]: אלה הדברים אשר ציוה לנו רבינו אשר ללמד לנו נתיב הישר ( מתוך ספר .... שחיבר חכם א' הנקרא רפאל מנורצי בעיר וויניציאה בשנת של"ט [1579]) [מתוך ארחות חיים, נ סימנים בלבד]. דף [8,א-10,ב]: במה דסיים הראש ז"ל פתח אלישע. ואמר שמעו ... והאזינו לדברי החכם הרלב"ג [מתוך פירושו על מלכים ב, פרק יב]. במה שהודיע לנו ע"פ הדברים אשר דיבר אלישע הנביא (ואיידי דחביבי אתן טעם לשבח ואבאר לכל הדברים) ... דף [10,ב]: רשמתי רק איזה טעות ... תקוני הטעויות מתיחסים רק לפירושו על מסכת ברכות ובשולי העמוד קישוט. ח, [4], ט-יב דף: פירוש ר' אלישע למסכת ברכות. עו, קא-קח דף: פירוש הרא"ש למסכתות פאה-בכורים. דף קג,ב-קה,ב: >הגה"ה< ראה זה מה שמצאתי בכתיבת יד החכם מוהר"מ [ר'משה] חאגיז נר"ו ... בדין הגר אם יכול לומר ולאבותינו ... הסכמות: ר' יחזקאל ב"ר אברהם מבית קצנאלנפוגין, אלטונא המבורג-וואנזיבעק, יח אלול תצ"ה; ר' משה ב"ר שאול לבית קצנאילנבוגן, שוואבך, כא מנחם [אב] תצ"ה; ר' משה חאגי"ז [אלטונא], תצ"ה; ר' יעקב [עמדין] ב"ר צבי אשכנזי, אלטונא, בא [אלול] תצ"ה.