The merchant/mathematician Leonardo of Pisa, who was also known as Fibonacci (literally, "son of Bonacci"), was perhaps the only European mathematician of note during the Middle Ages. His father was a Pisan merchant who was at one time a customs officer for the North African city of Bugia (now Bejaia, Algeria). Fibonacci lived there as a child. He was tutored by an Islamic tutor and learned about Hindu-Arabic numerals and various mathematical advances of the Arabs. Later on, Fibonacci toured the Mediterranean and visited many of the area's centres of learning, including Egypt, Greece, Sicily, Syria, and Provence. From these visits he learned the mathematics of the scholars and the calculating schemes in use.

In 1202, he wrote the first of his four mathematical books, Liber Abaci (he would rewrite this book in 1228). In this book Fibonacci used the Hindu-Arabic number system and the digits 0–9, making this book the first significant introduction of this system to Christian Europe. Out of all of the systems of numerals that he had learned in his travels, he held the Hindu-Arabic numerals in the highest regard, discounting all others as erroneous. As he writes in Liber Abaci, "Sed hoc totum et algorismum atque arcus pictagore quasi errorem computavi respectu modi indorum." The Muslim influence is clearly demonstrated in his books by the fact that Fibonacci wrote the digits in descending order, and wrote mixed fractions with the fractional part first.

At first, Liber Abaci was not well-received, but as time went on Europeans began to realise the importance of the book and of the Hindu-Arabic numerals that the book uses.

Fibonacci is most famous for the Fibonacci
Sequence, a sequence that was named (in the 19^{th} century)
for him based on one of the problems in Liber Abaci.

Sources used (see bibliography page for titles corresponding to numbers): 37.