The Transmission Electron Microscope (TEM) allows the user to determine the internal structure of materials, either of biological or non-biological origin. Materials for TEM must be specially prepared to thicknesses which allow electrons to transmit through the sample, much like light is transmitted through materials in conventional optical microscopy. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope. Thus, TEMs can reveal the finest details of internal structure - in some cases as small as individual atoms. Magnifications of 350,000 times can be routinely obtained for many materials, whilst in special circumstances, atoms can be imaged at magnifications greater than 15 million times. For biological samples, cell structure and morphology is commonly determined whilst the localisation of antigens or other specific components within cells is readily undertaken using specialised preparative techniques. For non-biological materials, phase determination as well as defect and precipitate orientation are typical outcomes of conventional TEM experiments. Microstructural characterisation of non-biological materials, including unit cell periodicities, can be readily determined using various combinations of imaging and electron diffraction techniques. Images obtained from a TEM are two-dimensional sections of the material under study, but applications which require three-dimensional reconstructions can be accommodate by these techniques.