By the end of the 13th century the Slavonic duchies were weakened, and the Lithuanian dukes had no difficulties to acquire many of the old Rusian territories. In 1319 the Duke of the Great Duchy of Lithuania (GDL) Gedymin took Bierascie but soon returned it to Valyn Slavonic duke Andrej. In 1341 Lithuanians acquired Bierascie again. In 1349 the city was taken by the Polish king Kazimierz the Third, who signed a treaty in 1366 with the Lithuanian duke Keystut, according to which the city was returned to the GDL and since that time it became officially a city of the GDL. By the middle of the 14th century all Belarusian territories were included in the GDL. Slavonic lands constituted the major part of the GDL, and the Belarusian language was the official state language on the whole territory of the GDL.
The development of the city was often interrupted by the invasion of Teutonic crusaders supported by the Pope. The Pope viewed the crusaders as the mean to promote the catholic faith on the Lithuanian, Belarusian, Ukrainian and Russian territories. In 1379 Bierascie was plundered and burnt down by the crusaders, but the population of the city escaped to the castle which was not taken by the invaders. After the invasion the city was quickly restored. In 1385 the Kreva Union was signed between the GDL and Poland to resist the aggression from crusaders. The Kreva Union also contributed to the spread of the Polish influence on the Belarusian lands.
Bierascie was one of the most important trade centers of the GDL. Many Russian, Polish, German, Lithuanian merchants would come to the city. The population of the city (approx. 2,000 people) was involved in trade, craft and agricultural production. The city’s merchants traded with Western Europe where they would bring leather, fur, wood, grain. They bought salt, paper, silk in Western Europe. In 1380 the first trade center was built in Bierascie.
In 1390 Bierascie, first of the Belarusian cities, was granted the Magderburg right of self government which was extended many times (in 1408, 1511, 1554, 1580, 1607, 1614, 1661). According to the Magderburg right the city governing was executed by a council (rada). The council was comprised of deputies, advisers and two city mayors. One of the mayors was Rusian, the other was Lithuanian, and the mayors rotated in heading the parliament. The members of the council were elected by the city residents and were coming from trade-craft beaumond. The council had big rights: it controlled trade activity and collected taxes. The head of the city council was voit appointed by the Duke of the Lithuanian State. The population of the city which did not belong to any feudal was under the jurisdiction of the city council and was taxed. Those people who belonged to any feudal rested under his master. The right of self governing contributed to the economic and cultural development of the city.
By the end of the 14th century the population of the city increased due to immigration of Jews from Poland and Germany. The Duke Vitaut granted Jews with a special order where he indicated civil and trade rights for them. The Jewish community in Brest was formed around 1390, and it constituted a fairly big part of the city population until W.W.II when it was wiped out by the Nazis. Representatives of the community were involved in trade and banking and were one of the most richest city residents. The territory of the city expanded in the beginning of the 15th century.
Further economic development of the city was halted by the aggression of the crusaders of the Teutonic Order against Lithuania and Poland. It was necessary for both countries to unite against the agressor. In the beginning of December of the year of 1409 the Lithuanian Duke Vitaut and the Polish King Yagaila converged to Bierascie to develop a plan against the enemy. Bierascie offered a detachment which participated in the Grunwald battle in July 15, 1410. The united Polish and Lithuanian army destroyed the crusaders, and they could never restore their might again.
Since the first part of the 15th century the economic and cultural situation of Bierascie was constantly improving. During the last decades 40 kinds of trade were developed in the city including shoemaking, forgery, clothmaking. The population of the city comprised 5 thousand people, artisans being the most numerous part of the city population. Being strong economically Bierascie became also an influential political center. The city hosted 17 seatings of the State Parliament (Sojm) during the period from 1446 till 1569. The city was part of the Trock vajavodstva. Even the invasion of Crimean Tatars led by Khan Mengli-Girei in 1500 did not halt the development of Bierascie. After the invaders left, the city was quickly restored. Since 1511 two annual fairs were held in the city, each of them continuing for about a month. The locally produced goods (bread, flax, linseed oil, leathers, sheepskins, tar) were bought by merchants, brought to fairs held in Gdansk (baltic shore) and sold there. Clothes, leather goods, coarse clothes were brought from Gdansk in exchange. In 1551 the Duke Sigizmund -August issued an order which stated that incomes from the city residents' plots should go into the city’s treasury. This money had to be spent to build an horologe and hire a trumpeter. In 1554 Bierascie was granted coat of arms which depicted a stone tower on the red background and confluence of two rivers. In 1559 Radzivil founded a printing house and in 1563 "The Brest Bible" was printed there. In 1569 first drug store and in 1590 first secular school appeared in the city. Data from audit books of 1566 confirm the expansion of the city territory. Its appearance also changed. The city castle was encircled by a wall with five towers on it. One of the towers had horologe. Three roads were leading to the city. Behind the wall and the towers, the buildings of city council, court, church, monastery, fair, and houses of wealthy residents were located. The castle hosted also water sawmill, two mills, brewery and water supplying mills through which water was supplied to the castle. At the city wall, suburbs spread where artisans, petty bourgeois and the poor were living. There were 930 houses in the city. The streets were paved with wood. First mentioning about stone paved streets refers to 1588. The population of the city comprised 8 thousand people at the end of the 16th century. The most numerous part of the population was represented by merchants and artisans. In the 16th century Bierascie had commercial relations with north-eastern Belarusian and north-western Ukrainian territories. Active trade was carried with Belarusian cities Slucak, Miensk, Mahileu. The merchants of Bierascie also traded with the Polish cities Warsaw, Poznan, Torun, Lublin. As it can be inferred, Bierascie that time was an important trade center of the GDL. According to customs data, in 1583 only during three days of the month of February 52 strings of carts (374 carts) passed through the city. The merchants would stop in the city for a while and trade some of their goods. All this contributed to the economic growth of Bierascie.