  The article presents a bundle framework for nonlinear observer design on a
manifold with a Lie group action. The group action on the manifold decomposes
the manifold to a quotient structure and an orbit space, and the problem of
observer design for the entire system gets decomposed to a design over the
orbit (the group space) and a design over the quotient space. The emphasis
throughout the article is on presenting an overarching geometric structure; the
special case when the group action is free is given special emphasis. Gradient
based observer design on a Lie group is given explicit attention. The concepts
developed are illustrated by applying them on well known examples, which
include the action of $\mathbb{SO}(3)$ on $\mathbb{R}^3 \setminus \{0\}$ and
the simultaneous localisation and mapping (SLAM) problem.
