The article deals with the Arabic sources of Chr. Clavius in Rome and the six different ways they were used by him in mathematics and astronomy. It inquires especially into his attitude towards al-Farghānī, Thābit ibn Qurra, al-Bi[tdotu]rūjī, Ibn Rushd, Mu[hdotu]ammad al-Baghdādī, Pseudo-Ibn al-Haytham, Jābir ibn Afla[hdotu], and Pseudo-al-[Tuotu]ūsī.