  This thesis examines linearly edge-reinforced random walks on infinite trees.
In particular, recurrence and transience of such random walks on general
(fixed) trees as well as on Galton-Watson trees (i.e. random trees) is
characterized, and shown to be related to the branching number of these trees
and a so-called reinforcement parameter. A phase transition from transience to
recurrence takes place at a critical parameter value. As a tool, random walks
in random environment are introduced and known results are repeated, together
with detailed proofs. A result on quasi-independent percolation is proved as a
by-product. Finally, for the edge-reinforced random walk on Z, the existence of
a kind of stationary / limiting distribution with finite moments is shown.
