  We study the structure of collections of algebraic curves in three dimensions
that have many curve-curve incidences. In particular, let $k$ be a field and
let $\mathcal{L}$ be a collection of $n$ space curves in $k^3$, with
$n<\!\!<(\operatorname{char}(k))^2$ or $\operatorname{char}(k)=0$. Then either
A) there are at most $O(n^{3/2})$ points in $k^3$ hit by at least two curves,
or B) at least $\Omega(n^{1/2})$ curves from $\mathcal{L}$ must lie on a
bounded-degree surface, and many of the curves must form two "rulings" of this
surface.
  We also develop several new tools including a generalization of the classical
flecnode polynomial of Salmon and new algebraic techniques for dealing with
this generalized flecnode polynomial.
