We study the spin dynamics of the 2-dimensional spin 1/2 Heisenberg model at finite temperature T, in particular the frequency region omega < T much less than J (the coupling constant). The main conclusion reached is that the spin fluctuations have relaxational character, no propagating spin waves exist for omega < T. Comparison with experimental results on La2CuO4 give good agreement. Similarities and differences to previous theoretical treatments are discussed.