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Key issues of classical and quantum strings in gravitational plane waves,shock waves and spacetime singularities are synthetically understood. Thisincludes the string mass and mode number excitations, energy-momentum tensor,scattering amplitudes, vaccum polarization and wave-string polarization effect.The role of the real pole singularities characteristic of the tree level stringspectrum (real mass resonances) and that of spacetime singularities is clearlyexhibited. This throws light on the issue of singularities in string theorywhich can be thus classified and fully physically characterized in twodifferent sets: strong singularities (poles of order equal or larger than 2,and black holes), where the string motion is collective and non oscillating intime, outgoing and scattering states do not appear, the string does not crossthe singularities, and weak singularities (poles of order smaller than 2, Diracdelta, and conic/orbifold singularities) where the whole string motion isoscillatory in time, outgoing and scattering states exist, and the stringcrosses the singularities. Commom features of strings in singular planebackgrounds and in inflationary backgrounds are explicitly exhibited. Thestring dynamics and the scattering/excitation through the singularities(whatever their kind: strong or weak) is fully physically consistent andmeaningful.Comment: Synthesis and new material. 18 pages, no figure

CLASSICAL AND QUANTUM STRINGS IN PLANE WAVES, SHOCK WAVES AND SPACE–TIME SINGULARITIES: SYNTHESIS AND NEW RESULTS