  We consider a class of quasilinear wave equations in $3+1$ space-time
dimensions that satisfy the "weak null condition" as defined by Lindblad and
Rodnianski \cite{LR1}, and study the large time behavior of solutions to the
Cauchy problem. The prototype for the class of equations considered is
$-\partial_t^2 u + (1+u) \Delta u = 0$. Global solutions for such equations
have been constructed by Lindblad \cite{Lindblad1,Lindblad2} and Alinhac
\cite{Alinhac1}. Our main results are the derivation of a precise asymptotic
system with good error bounds, and a detailed description of the behavior of
solutions close to the light cone, including the blow-up at infinity.
