Crossover between BCS and preformed-boson theories with increasing interactions

I review recent work on Fermi systems with attractive interactions of arbitrary strength. First, I describe the results of a functional integral formulation to study the crossover from cooperative Cooper pairing to independent bound state formation and condensation. The inadequacy of a saddle point approximation with increasing coupling is pointed out, and the importance of temporal (quantum) fluctuations for normal state properties at intermediate and strong coupling is emphasized. Next, results from a quantum Monte-Carlo simulation of the two-dimensional (2D) attractive Hubbard model are described. The intermediate coupling normal state is found to deviate markedly from a canonical Fermi liquid: ''spin-gap'' behavior, with 1/T1T similar χ(T), is found in a degenerate Fermi system. ((orig.))