Galileo Galilei was born in the city of Pisa, in the northern Italian region of Tuscany, on February 15, 1564. His father was Vincenzo Galilei, an accomplished musician who worked as a wool trader in order to make enough to money to attain the aristocratic lifestyle desired by his wife, Giulia Ammananti. Galileo's mother came from a higher social station than her husband, and she has been described as a shrewish, demanding woman, ever resentful of Vincenzo's failure to rise above his lower-class origins. She seems to have harbored an abiding objection to his pursuit of music, but Vincenzo continued to experiment with his favored instrument, the lute, throughout Galileo's childhood, even as his wife gave birth to six more children. When Galileo was four, his father published a brief book celebrating the lute's superiority to the organ, and in 1572, Vincenzo temporarily deserted his family to take his place in a circle of artists and musicians in Florence, the principal city of Tuscany, and the seat of the ruling grand duke, Cosimo de Medici.

Vincenzo's primary interest lay in the recovery of ancient Greek and Latin musical forms, an aspiration that made him very much a man of his times. Galileo was born into the waning decades of the Italian Renaissance, that explosion of arts and letters in the 15th and 16th centuries that stressed the recovery and adaptation of classical art and philosophy lost since the fall of the Roman Empire a thousand years before. At its best, Renaissance Italy stirred a seething cauldron of artistic and intellectual ferment, in which geniuses like Michelangelo, Machiavelli, Da Vinci, and Petrarch fashioned a new humanism for a newly modern Europe. But it was also a corrupt time and place, where greedy local princes dueled with the French and Spanish for control of a patchwork quilt of city-states.

These local princes were also dueling with the popes; for during these years the popes served not only as the spiritual heads of the Roman Catholic Church, but also as secular rulers, dominating large swathes of territory in central Italy, and playing politics as well as any Italian prince or duke. The Renaissance Popes were a colorful group who acted as great patrons of the flourishing artistic culture, but whose holiness left something to be desired. The reign of the notorious Alexander VI (1492-1503) had marked the Church's moral low-point: Alexander kept a number of mistresses and schemed, unsuccessfully, to make his illegitimate son ruler of all Italy.

By Galileo's youth, the Church was well on its way to ridding itself of the excesses of Alexander and his fellows, but their efforts came too late–a reaction had already broken out in northern Europe, led by a former monk named Martin Luther. This movement, called the Protestant Reformation, soon swept through Germany, Scandinavia, and eventually England. While it has often been characterized as a liberal reaction against Catholic conservatism, the opposite was in fact the case: Martin Luther and his fellow Protestants attacked the Church for having become too worldly and politically corrupt, and for obscuring the fundamentals of Christian faith with pagan elements ranging from the cult of the saints to the adoration of the Virgin Mary. Their reforming zeal was essentially reactionary–they appealed to a notion of an original, "purified" Christianity, and their austerity of worship, with its contempt for religious art and music, constituted a direct repudiation of the Renaissance spirit. Of course, this notion of a "purer" faith did not always manage to avoid hypocrisy: in order to escape the powerful suppression of popes, the new Protestant churches often had to forge alliances with the German princes and English kings, becoming little more than arms of the rising nation-states of Europe.

Meanwhile, Roman Catholicism was forced to adopt a warlike position against this new faith threatening its existence. Thus the Church formed a Council of Trent, which in turn called for a Counter-Reformation, emphasizing orthodoxy and fidelity to the true Church. A new religious order called the Jesuits, or Society of Jesus, arose to embody the spirit of this movement, and formed the vanguard of the battle with Protestant heresy. The Counter-Reformation benefited Catholicism in many ways, as it reinvigorated a flabby Church, and produced a great wave of intellectual, artistic, and religious energy (in Spain, especially) embodied by saints like Francis Xavier, Ignatius of Loyola, and Teresa of Avila. But it also put an end to the lenient spirit of the Renaissance, and its emphasis on religious orthodoxy, rigidly enforced by the Inquisition, would clash with the new spirit of scientific inquiry that was soon to stir the young Galileo's imagination.

However, this clash still lay in the future when in 1574, the entire Galilei family moved to Florence and re-united with Vincenzo. There, Galileo's father took his oldest son under his wing, and seeing great intellectual potential in the boy, sent Galileo to the monastery of Vallombrosa, twenty miles east of Florence, to receive a full education in the humanities. The scholarly bent of the community appealed to Galileo, and in his fourth year at Vallombrosa, he informed his father that he intended to become a monk. His father, who had never held the Church in great esteem, responded by withdrawing him from the monastery and formulating a new plan for his education. He decided that Galileo would return to Pisa, to enroll in the university there, while receiving training in the wool business from a cousin, as a university education did not guarantee financial success. As Galileo became more ardent in his own pursuits and individual interests, he would abandon this formal education. But meanwhile, in late summer of 1581, the young man entered the University of Pisa, to study for a degree i n medicine.