  We present numerical exact results for the ground state and the low-lying
excitations for the spin-1/2 J1-J2 Heisenberg antiferromagnet on finite square
lattices of up to N=40 sites. Using finite-size extrapolation we determine the
ground-state energy, the magnetic order parameters, the spin gap, the uniform
susceptibility, as well as the spin-wave velocity and the spin stiffness as
functions of the frustration parameter J2/J1. In agreement with the generally
excepted scenario we find semiclassical magnetically ordered phases for J2 <
J2^{c1} and J2 > J2^{c2} separated by a gapful quantum paramagnetic phase. We
estimate J2^{c1} \approx 0.35J1 and J2^{c2} \approx 0.66J1.
