Very small things on the order of micrometers/smaller that cannot be resolved with a light microscope. Things like viruses, non mitotic DNA, certain bacteria/cells can only be resolved via an electron microscope. Also all these things must be dead first and coated w/ a metal such as gold. Therefore in can't be used to study living organisms. Finally an electron microscope specifically a scanning electron microscope is used to study the surface of larger items w/ very high resolution.
