Historically the foundations of the contemporary study of two-magnon states were laid by Wortis1 in 1963 when he located the two-magnon bound states of the n.n. isotropic hypercubic Heisenberg ferromagnet, established the connection with bound states of the corresponding Ising problem and gave a spin Green function formalism that exactly described the 2-magnon problem in ferromagnets at T=0°K. The conclusion that 2-magnon bound states in the n.n. s.c. case only occurred at large values of the total pair wavevector \( \left( {\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {K} = {{\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {K} }_1} + {{\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {K} }_2}} \right) \) and with an energy too large to affect the low temperature thermodynamics did not offer much hope for observing these effects. The scene shifted to broad peaks detected in optical studies of transparent antiferromagnets2 in 1966 (infrared absorption, Raman scattering) which stimulated \( \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {K} = 0 \) calculations that eventually attained good agreement with experiment when magnon-magnon interactions were properly included by Elliott and Thorpe3. Even at T=0°K the antiferromagnetic formulation is subject to decoupling approximations in contrast to the ferromagnet.