The transmission electron microscope operates on the same basic principles as the light microscope but uses electrons instead of light to illuminate the sample. The very short wavelength of accelerated electrons improves the resolution to the order of 0.2 nm (0.0000002 mm) which is a thousand times better than what can be resolved by a light microscope.

The high magnifications and the analytical possibilities has made the TEM an indispensable tool in both medical, biological and materials research.

The diverse needs of different applications have resulted in many different models. In general we can group the transmission electron microscopes into 4 groups according to the maximum accelerating voltage, which also influences both size and cost.

100 - 120 kV TEMs
These are microscopes mainly used to study biological samples. The difficult sample preparations and the sample's sensitivity to electron bombardment often put severe limitations on the maximum useful magnification.  Factors like ease of use, cost, contrast and flexibility are often more important than the ultimate attainable resolution.

200 kV TEMs
These TEMs are mainly used for material research and advanced macromolecular research. The higher accelerating voltage means better resolution and better penetration, which makes 200 kV instruments ideal for work on metals, ceramics and semiconductor materials.

These instruments are also available with field emission electron emitters instead of the normal lanthanum hexaboride  emitter. Apart from the increased beam coherence, such instruments can also generate a focused electron beam of atomic size intense enough to allow X-ray or electron energy spectroscopy from areas less than 1 nm in size.

300 kV TEMs
These are basically big brothers to the 200 kV TEMs giving further improved resolution, penetration and analytical performance.

400 - 1,200 kV TEMs
These are the really large instruments for institutes specializing in studies requiring the ultimate in resolution on atomic scale.