The transmission electron microscope (TEM) was invented in the 1930's, and developments in specimen preparation in the 1950's led to its widespread use as a tool to study structure in biologic systems. Similar in principle to the light microscope, but utilizing a much shorter wavelength for better resolution, the TEM has the image-forming beam pass through the specimen. This results in a two-dimensional image which can be difficult to interpret because features from different depths of the three dimensional specimen are superimposed. Traditionally this was dealt with by cutting sections of plastic-embedded specimens so thin (in the 40 to SO nanometer range) that they effectively had only two dimensions. To allow biologists to examine structures in three dimensions, serial sections are stacked and structures reconstructed. Even though computers have made reconstruction easier, the reality is that resolution in the depth dimension is limited by the section thickness. The technique of electron tomography is emerging as a way to overcome this limitation.