  Given an arbitrary countable directed graph $G$ we prove the C*-envelope of
the tensor algebra $T_+(G)$ coincides with the universal Cuntz-Krieger algebra
associated with $G$. Our approach is concrete in nature and does not rely on
Hilbert module machinery. We show how our results extend to the case of higher
rank graphs, where an analogous result is obtained for the tensor algebra of a
row-finite $k$-graph with no sources.
