The geometry of graded principal bundles is discussed in the framework
of graded manifold theory of Kostant-Berezin-Leites. We first review
the basic elements of this theory establishing at the same time supple
mentary properties of graded Lie groups and their actions. Particular
emphasis is given in introducing and studying free actions in the grad
ed context. Next, we investigate the geometry of graded principal bund
les; we prove that they have several properties analogous to those of
ordinary principal bundles. In particular, we show that the sheaf of v
ertical derivations on a graded principal bundle coincides with the gr
aded distribution induced by the action of the structure graded Lie gr
oup. This result leads to a natural definition of the graded connectio
n in terms of graded distributions; its relation with Lie superalgebra
-valued graded differential forms is also exhibited. Finally, we defin
e the curvature for the graded connection and we prove that the curvat
ure controls the involutivity of the horizontal graded distribution co
rresponding to the graded connection.