Cell theory states that all living organisms are made up of one or more number of cells, a cell is the basic structural or functioning unit of all organisms and also all cells come from pre-existing cells. Modern microscopes supports cell theory.
LIGHT MICROSCOPES: in light microscopes the visible light passes through the biological specimen and it bends through the lens system allows to observe magnified images. light microscopes allows observation of living cells which are in their dividing stage.
PHASE CONTRAST MICROSCOPES: this technique allows the observation of unstained living cells in detail. Phase-contrast microscopy is an optical microscopy technique that reveals cell structure  that are not visible through light microscopes.
QUANTITATIVE PHASE CONTRAST MICROSCOPES: is an alternative microscopic method for living cell observation. It has an advantage over fluorescent and phase contrast microscopy because this is non-invasive and quantitative in nature.
FLUORESCENCE MICROSCOPES: Â these microscopes utilizes the property of fluorescence which means that a cell absorbs one wavelength of light and emit another wavelength. Light of smaller wavelengths are used to excite fluorescent molecules which result in the emission of light of longer wavelengths which is collected to form the image of cells of the specimens. Now, commercially available fluorescent probes allows structural and physiological studies of cells and tissues useful for those specimens which does not show natural fluorescence property.
Fluorescence correctional spectroscopy (FCS) and fluorescence cross- correlation spectroscopy (FCCS) these are the advance techniques to reveal the size and dynamics of macromolecules in the living cells and tissues these techniques uses specialized spectroscopy and imaging systems.
CONFOCAL MICROSCOPES: these microscopes uses laser beams to excite the molecules of the sample collects the emitted light and gives the sharper image than conventional fluorescence microscopes.These microscopes are commonly used. They are used to study living cells and tissue preparations.
ELECTRON MICROSCOPES: the microscopes provide very high resolution. Electron microscopes are different from light microscopes as they produce the image  of a specimen by using the beam of electrons rather than beam of light. These microscopes not only give the image of cells but also includes interior of all the cells i.e the cell organelles.
There are two major types of electron microscopy.
 Scanning electron microscopy (SEM) :  a beam of electrons moves back and forth across the surface of a cell or tissue, creating a detailed image of the 3D surface. This type of microscopy was used to take the image of the Salmonella bacteria shown at right, above.
Transmission Electron Microscopy (TEM) : in this technique the sample is cut into thin slices  and electron beam passes into each slice rather skimmimg over the external surface of cellular layer.
