New solid forms of fesoterodine fumarate are described. In particular, amorphous fesoterodine fumarate, characterized by a powder X-ray diffraction spectrum as shown in FIG. 1 and by an IR spectrum as shown in FIG. 2 is described. Also fesoterodine fumarate in crystalline form I, characterized by a powder X-ray diffraction spectrum as shown in FIG. 3, by a DSC profile as shown in FIG. 4, by an IR spectrum as shown in FIG. 5, by a solid state 13C-NMR spectrum as shown in FIG. 6 and by a Raman spectrum as shown in FIGS. 7, 8 and 9 is described.