  I calculate the effect of interactions among superconducting quasiparticles
in two-dimensional(2D) a superconductor at T=0. The strength of the effective
interaction among the quasiparticles is essentially given by the screened
Coulomb interaction which has strength at low frequency because of the gapless
nature of the plasmon. This is in contrast to three dimensions where the
effective interaction has negligible weight at frequencies $\sim \Delta$, the
superconducting gap. The quasiparticle interactions give rise to
strong-coupling effects in experimental quantities which are beyond the
conventional Eliashberg treatment of superconductivity. The present calculation
offers an explanation of why these effects are much larger in 2D than in 3D
superconductors and, in particular, why the analogous strong-coupling effects
due to quasiparticle interactions are seen in data on the quasi-2D cuprate
superconductors. the strong-coupling features seen in data on the cuprates are
discussed in light of the present calculation.
