On February 15, 1564 Galileo was born in Pisa, the son of Vincenzo Galilei, a music teacher who came from Florence, and Giulia Ammannati, from a Pescia family that had moved to Pisa years earlier. He was born at the residence of his uncle, Leone Ammannati, a house belonging to the church of S. Andrea Forisportam, as emerges from his birth certificate drawn up on February 19 in the Baptistery of Pisa.

Vincenzo Galilei was obliged to move to Florence, perhaps to engage in some commercial activity in tandem with his work as a musician, leaving his family in Pisa in the care of his friend Muzio Tedaldi, who was later to marry Giulia's niece. The young Galileo began his education at the public school in Pisa, probably between 1569 and 1574. The school appointed for three-year periods masters of writing, grammar and arithmetic, obliging them by contract to find suitable accommodation for teaching and, according to a document in the State Archives of Pisa, to teach 'all equally, the poor citizens as well as the rich.' Galileo may have learned here the first elements of Greek, as Antonio Leonardi da Castiglione, a master of grammar during his years as a pupil, was one of the few to be employed as magister literarum graecarum.

Towards the end of 1574 Galileo moved to Florence to join his father. He remained there for some years, extending his studies of 'the humanities, Greek and dialectics', as well as drawing and music (he was, it seems, a good lute player). According to Niccolò Gherardini, a biographer who in truth knew little about Galileo's youth, he was sent 'to the school of a teacher of grammar, a very undistinguished man, who taught in his own house located in the Via de' Bardi.' The Galilei family possibly lived nearby, since, at the foot of a letter from Muzio Tedaldi to Galileo's father, we find the statement 'consigned to Pier Francesco Lapini, living across from the Torrigiani hill', behind the Palazzo de' Mozzi in the Oltrarno, adjoining the Via de' Bardi.

Galileo then continued his studies with the Vallombrosan monks - whether in the monastery of Vallombrosa or in the community of Santa Trinita, as Viviani states, we do not know - and entered the order, it seems, as a novice. Dating from this period are several notes in his hand relating to Aristotelian logic, probably reflecting tuition modelled on that of the Jesuits in the Collegio Romano. His father however did not allow him to complete the course of study, 'on the pretext', the Vallombrosan Abbot Diego Franchi insinuated, 'of taking him to Florence to treat a severe eye condition.' This was in 1578. In 1580 Galileo returned to Pisa to enroll at the university as artista, that is, student of medicine and philosophy, living once again under the wing of Muzio Tedaldi as a guest in his house.