Whereby semiconductor device pitch is bounded by physical limits such as illumination wavelength and numerical aperture, the ability to control the relative placement of features between layers faces no such fundamental constraint.  While the requirements on lithographic pitch reduction have slowed in recent nodes, the layer to layer lithographic overlay control budget has continued to shrink monotonically.  Subsequently, overlay metrology performance demands in terms of precision and accuracy continue to shrink exponentially over time.  Unchanged is the fundamental reliance on principles of symmetry and symmetry breaking which are at the root of the real and pupil image based metrology techniques in use today.  The theoretical underpinnings of these two metrology paradigms will be briefly presented and the similarities and differences highlighted.  The concept of the metrology landscape will be introduced, an important analytical tool, whether in the service of measurement condition or metrology structure optimization.   The lecture will be punctuated with measured and simulated visualizations of real and pupil images alongside spectral and focal landscapes.