The need for a Polish-Lithuanian union arose in the late fourteenth century around the threat posed to both the Crown and the Grand Duchy by the expansion of the Teutonic Order. The baptism of Lithuania was meant to deny the Teutonic order any pretext for invading the country, to provide the basis for an alliance between Poland and Lithuania, and buttress the rule of the Lithuanian magnates over the Ruthenian population, which was numerically predominant in the Duchy.

Together with all the people of his lands, Jagiełło chose to accept baptism in the Roman Catholic rite, release all Christian captives from Lithuanian bondage, strive to regain the lands lost by the two countries, and also to adjoin [applicare in the Latin text] his Lithuanian and Ruthenian territories to the Crown of the Kingdom of Poland. The latter promise became the seedbed of a dispute between Jagiełło and his first-cousin Witold (Vytautas), who ruled in Lithuania. Moreover, in the following decades this dispute embroiled the Polish and Lithuanian lords, as well. To this day the interpretation of the word applicare remains a subject of controversy between Polish and Lithuanian historians. Polish scholars read the term as describing the intention to incorporate the lands of the Grand Duchy of Lithuania into the Polish Crown, while Lithuanian scholars argue that it expressed the clear intention of maintaining a loose federation of independent states, such as typified the Polish-Lithuanian union until the year 1569.

The Statute of Piotrków, containing the laws of Greater Poland, was composed between 1356 and 1362, and announced at a gathering in Piotrków. It was anterior to the statutes concerning Lesser Poland, promulgated at the gathering in Wiślica, in 1362 at the latest.

The statutes contain provisions concerning both the state system and judicial law (especially criminal law). They were not codifications in the modern sense, because they did not provide an exhaustive overview of any section of law, but they introduced, following canon law, the superiority of the established norm over the custom, which became an auxiliary norm.

Of the more interesting regulations relating to the social system, it is worth mentioning the punishment for murder (główszczyzna) and the Test of Nobility, which had to do, on the one hand, with the rising prominence of the knighthood (nobility) in public life and, on the other hand, with the lack of a specialised heraldic office controlling who belonged to the nobility. The Statute of Wiślica also confirmed the military service duty of knights, subject, however, to the privilege of being remunerated for serving abroad, which the knighthood had obtained as early as the thirteenth century.

Despite their incompleteness, the statutes of Casimir the Great played an important role in constructing a united kingdom and in amending existing legal norms. In the fifteenth century, the statutes of Piotrków and Wiślica were merged into one.