We have carried out a neutron scattering study of the instantaneous spin-spin correlations in La2CuO4 (TN = 325 K) over the temperature range 337 K to 824 K. Incident neutron energies varying from 14.7 meV to 115 meV have been employed in order to guarantee that the energy integration is carried out properly. The results so-obtained for the spin correlation length as a function of temperature when expressed in reduced units agree quantitatively both with previous results for the two dimensional (2D) tetragonal material Sr2CuO2Cl2 and with quantum Monte Carlo results for the nearest neighbor square lattice S=1/2 Heisenberg model. All of the experimental and numerical results for the correlation length are well described without any adjustable parameters by the behavior predicted for the quantum non-linear sigma model in the low temperature renormalized classical regime. The amplitude, on the other hand, deviates subtly from the predicted low temperature behavior. These results are discussed in the context of recent theory for the 2D quantum Heisenberg model. The physics of low dimensional quantum Heisenberg antiferromagnets has been the subject of research ever since the advent of modern quantum and statistical mechanics [1,2]. Interest in two dimensional (2D) systems was heightened by the discovery of high temperature superconductivity in the lamellar copper oxides [3]. Specifically, it was realized early on that the parent compounds such as La 2 CuO 4 correspond to rather good approximations to the S=1/2 2D square-lattice quantum Heisenberg antiferromagnet (2DSLQHA) [4,5]. It seems at least possible that the 2D magnetism may in some way be essential to the superconductivity in the charge carrier doped cuprates. Further, the magnetism itself is of fundamental interest as a quantum many body phenomenon in lower dimensions.Early experiments by Endoh and co-workers [5] showed that over a wide range of temperatures above the three dimensional Néel ordering transition in La 2 CuO 4+y (that is, La 2 CuO 4 with a small amount of excess oxygen) the instantaneous spin-spin correlations were purely two dimensional and that the correlation length diverged exponentially in 1/T. This led to a flurry of theoretical activity [2] including most especially work based on the quantum non-linear sigma model (QNLσM) by Chakravarty, Halperin and Nelson (CHN) [6] and Hasenfratz and Niedermayer (HN) [7]. These theories are all based on the 2D Heisenberg Hamiltonian which for nearest neighbor (nn) interactions alone takes the formwhere the summation is over nn pairs on a square lattice. In La 2 CuO 4 , for temperatures below the tetragonal (I4/mmm) -orthorhombic (Bmab) structural phase transition temperature of T st = 530 K, the leading terms in the spin Hamiltonian [8,9] areHere, α nnn , α xy , α ⊥j , and α DM represent the reduced next nearest neighbor in-plane Heisenberg exchange coupling, XY anisotropy, interlayer coupling and Dzyaloshinski-Moriya antisymmetric exchange, respectively, and S c i is the c component of the spin at site...