  A tropical curve \Gamma is a metric graph with possibly unbounded edges, and
tropical rational functions are continuous piecewise linear functions with
integer slopes. We define the complete linear system |D| of a divisor D on a
tropical curve \Gamma analogously to the classical counterpart. We investigate
the structure of |D| as a cell complex and show that linear systems are
quotients of tropical modules, finitely generated by vertices of the cell
complex. Using a finite set of generators, |D| defines a map from \Gamma to a
tropical projective space, and the image can be extended to a tropical curve of
degree equal to \deg(D). The tropical convex hull of the image realizes the
linear system |D| as a polyhedral complex. We show that curves for which the
canonical divisor is not very ample are hyperelliptic. We also show that the
Picard group of a \Q-tropical curve is a direct limit of critical groups of
finite graphs converging to the curve.
