This section will provide a background for understanding and visualizing the microstructures observed in LDMMs. The word microcellular means small cell size, or more generally (because most low-density materials are not composed of easily recognizable “cells”) some measure of the spatial scale of the foam structure. The concept, measurement, and significance of cell size is intrinsically connected to the morphology, as are certain properties (e.g., stiffness). These matters are covered in later sections. As a background, this article describes some of the structural morphologies of low-density foams, not all of them available as LDMMs (i.e., simultaneously low-density and microcellular.) Interested readers are encouraged to also examine References 1 and 2 because a detailed discussion would take us too far afield. Instead, we shall indicate how most low-density morphologies may be considered in terms of a few simple models of morphological types encountered and how they arise. Often these are seen most cleanly in large-celled foams. Note that these are simplifications—there is no standard nomenclature.