This thesis consists of two papers on different but related topics.The first paper is concerned with the use of logic as a tool to model mechanical assembly processes. A restricted 2+-dimensional world is introduced and although this world is considerably simpler than a 3-dimensional one, it is powerful enough to capture most of the interesting geometrical problems arising in assembly processes. The geometry of this 2+-dimensional world is axiomatized in first order logic. A basic set of assembly operations are identified and these operations are expressed in a variant of dynamic logic which is modified to attack the frame problem.

The second paper presents a formalism for reasoning about systems of sequential and parallel actions that may interfere or interact with each other. All synchronization of actions is implicit in the definitions of the actions and no explicit dependency information exists. The concept of action hierarchies is defined, and the couplings between the different abstraction levels are implicit in the action definitions. The hierarchies can be used both top-down and bottom-up and thus support both planning and plan recognition in a more general way than is usual.