A continuous process for preparing polybutylene terephthalate, comprising: (a) combining 1,4-butane diol and purified terephthalic acid in a slurry paste vessel to form a mixture; (b) continuously supplying the mixture from step (a) to a tower reactor having a plurality of reactor zones for at least one of esterification or transesterification; wherein the following conditions are maintained: the mixture from step (a) is subjected to the esterification section with a treatment temperature in the range of 170 to 270 °C and a treatment pressure in the range of 0.5 to 1 bar; a first quantity of catalyst is supplied; the product of step is transferred continuously into a pipe stretch while optionally a quantity of BDO is supplied; the product of step is transferred continuously to a cascade postesterification part of the tower reactor, comprising multiple cascades in series, preferably four cascades in series, wherein the pressure of each cascade is subsequently reduced to ultimately ≤0.25 bar, preferably ≤ 0.20 bar and optionally a second quantity of catalyst is supplied into the last cascade zone of the post-esterification part of the tower reactor; (c) the product obtained from step is continuously supplied to a first continuously stirred tank reactor, wherein the product of step is subjected to a melt temperature of 225 to 250 oC, a pressure of 5 to 40 mbar, and a residence time between 10 and 60 minutes; (d) optionally, the product obtained from step (c) is continuously supplied to a second continuously stirred tank reactor, wherein the product of step (c) is subjected to a melt temperature of 230 to 260 oC, a pressure of 0.1 to 35 mbar, and a residence time between 10 and 60 minutes; and (e) the obtained product from step (c) or, where a second continuously stirred tank reactor (d) is used, from step (d), is continuously transferred to a continuous polycondensation reactor wherein the product of step (d) is subjected to a melt temperature of 230 to 255 oC, a pressure of 0.1 to 16 mbar, and a residence time of 30 minutes to 6 hours. (a) combining 1,4-butane diol and purified terephthalic acid in a slurry paste vessel to form a mixture; (b) continuously supplying the mixture from step (a) to a tower reactor having a plurality of reactor zones for at least one of esterification or transesterification; wherein the following conditions are maintained: the mixture from step (a) is subjected to the esterification section with a treatment temperature in the range of 170 to 270 °C and a treatment pressure in the range of 0.5 to 1 bar; a first quantity of catalyst is supplied; the product of step is transferred continuously into a pipe stretch while optionally a quantity of BDO is supplied; the product of step is transferred continuously to a cascade postesterification part of the tower reactor, comprising multiple cascades in series, preferably four cascades in series, wherein the pressure of each cascade is subsequently reduced to ultimately ≤0.25 bar, preferably ≤ 0.20 bar and optionally a second quantity of catalyst is supplied into the last cascade zone of the post-esterification part of the tower reactor; the mixture from step (a) is subjected to the esterification section with a treatment temperature in the range of 170 to 270 °C and a treatment pressure in the range of 0.5 to 1 bar; a first quantity of catalyst is supplied; the product of step is transferred continuously into a pipe stretch while optionally a quantity of BDO is supplied; the product of step is transferred continuously to a cascade postesterification part of the tower reactor, comprising multiple cascades in series, preferably four cascades in series, wherein the pressure of each cascade is subsequently reduced to ultimately ≤0.25 bar, preferably ≤ 0.20 bar and optionally a second quantity of catalyst is supplied into the last cascade zone of the post-esterification part of the tower reactor; (c) the product obtained from step is continuously supplied to a first continuously stirred tank reactor, wherein the product of step is subjected to a melt temperature of 225 to 250 oC, a pressure of 5 to 40 mbar, and a residence time between 10 and 60 minutes; (d) optionally, the product obtained from step (c) is continuously supplied to a second continuously stirred tank reactor, wherein the product of step (c) is subjected to a melt temperature of 230 to 260 oC, a pressure of 0.1 to 35 mbar, and a residence time between 10 and 60 minutes; and (e) the obtained product from step (c) or, where a second continuously stirred tank reactor (d) is used, from step (d), is continuously transferred to a continuous polycondensation reactor wherein the product of step (d) is subjected to a melt temperature of 230 to 255 oC, a pressure of 0.1 to 16 mbar, and a residence time of 30 minutes to 6 hours.