The fluorescence excitation spectra and dispersed fluorescence spectra of trans-stilbene have been recorded and analyzed. Vibrational assignments for the eight low-frequency modes have been made for both the S-0 and S-1(pi,pi*) electronic states, and these differ substantially from those of previous workers. Two-dimensional kinetic and potential energy calculations were carried out in order to determine the potential energy surfaces for the two phenyl internal rotations v(37) and v(48). The function V(phi(1), phi(2))=1/2V(2)(2+cos 2 phi(1)+cos 2 phi(2))+V-12 cos 2 phi(1) cos 2 phi(2)+V’(12) sin 2 phi(1) sin 2 phi(2), with V-2=1550 cm(-1), V-12=337.5 cm(-1), and V’(12)=402.5 cm(-1) for the S-0 state and with V-2=1500 cm(-1), V-12=-85 cm(-1), and V’(12)=-55 cm(-1) for the S-1(pi,pi*) state fits the observed data (nine Frequencies for S-0 and six for S-1) extremely well. The barriers to simultaneous internal rotation of both phenyl groups are given by twice the V-2 values. The fundamental frequencies for these torsions are nu(37)=9 cm(-1) and nu(48)=118 cm(-1) for the S-0 state and nu(37)=35 cm(-1) and v(48)=110 cm(-1) for the S-1 excited state. The third torsion nu(35), which is the internal rotation about the C=C bond, was assigned at 101 cm(-1) for the S-0 state based on a series of overtone frequencies (202, 404 cm(-1), etc.). For S-1, nu(35) 99 cm(-1) based on observed frequencies at 198, 396 cm(-1), etc. Kinetic energy calculations were also carried out for this mode, and a one-dimensional potential energy function of the form V(theta)=1/2V(1)(1-cos theta)+1/2V(2)(1-cos 2 theta)+1/2V(4)(1-cos 4 theta) was utilized to reproduce the frequencies for the ground state. For the excited state, an additional V-8 term was added in order to fit the data for the trans potential energy well. The data indicate that the trans-->twist barrier for the S-1 state is higher than 1400 cm(-1). However, a somewhat revised frequency assignment would be compatible with a barrier of 1250 cm(-1), which is close to the value of 1200 cm(-1) determined from dynamics studies.