  This review focuses on flat and superthin galaxies. These are edge-on
bulgeless galaxies, which are composed of a simple, stellar disk. The
properties of these simple disks are at the end of a continuum that extends
smoothly from bulge-dominated disk galaxies to the pure disks. On average,
simple disks are low-mass galaxies with low surface brightnesses, blue colors,
and slow rotational velocities. Widely-accepted cosmological models of galaxy
formation and evolution were challenged by a relatively large observed fraction
of pure disk galaxies, and only very recent models can explain the existence of
simple disk galaxies. This makes simple disks an optimal galaxy type for the
study of galaxy formation in a hierarchical Universe. They enable us to analyze
the environmental and internal influence on galaxy evolution, to study the
stability of the disks, and to explain the nature and distribution of dark
matter in galaxies. This review summarizes the current status of edge-on simple
disk galaxies in the Universe.
