  The duality theory of the Monge-Kantorovich transport problem is investigated
in an abstract measure theoretic framework. Let $(\mathcal{X},\mathcal{F},\mu)$
and $(\mathcal{Y},\mathcal{G},\nu)$ be any probability spaces and
$c:\mathcal{X}\times\mathcal{Y}\rightarrow\mathbb{R}$ a measurable cost
function such that $f_1+g_1\le c\le f_2+g_2$ for some $f_1,\,f_2\in L_1(\mu)$
and $g_1,\,g_2\in L_1(\nu)$. Define $\alpha(c)=\inf_P\int c\,dP$ and
$\alpha^*(c)=\sup_P\int c\,dP$, where $\inf$ and $\sup$ are over the
probabilities $P$ on $\mathcal{F}\otimes\mathcal{G}$ with marginals $\mu$ and
$\nu$. Some duality theorems for $\alpha(c)$ and $\alpha^*(c)$, not requiring
$\mu$ or $\nu$ to be perfect, are proved. As an example, suppose $\mathcal{X}$
and $\mathcal{Y}$ are metric spaces and $\mu$ is separable. Then, duality holds
for $\alpha(c)$ (for $\alpha^*(c)$) provided $c$ is upper-semicontinuous
(lower-semicontinuous). Moreover, duality holds for both $\alpha(c)$ and
$\alpha^*(c)$ if the maps $x\mapsto c(x,y)$ and $y\mapsto c(x,y)$ are
continuous, or if $c$ is bounded and $x\mapsto c(x,y)$ is continuous. This
improves the existing results in \cite{RR1995} if $c$ satisfies the quoted
conditions and the cardinalities of $\mathcal{X}$ and $\mathcal{Y}$ do not
exceed the continuum.
