Genetic information is stored in the deoxyribonucleic acid (DNA) found in the nucleus. The biological functions of nucleic acids are ultimately coupled to their three-dimensional (3D) structures, either in the presence or absence of any interacting proteins. Diffraction methods are among the most powerful means of obtaining information about the 3D structures of nucleic acids and other biological molecules. The first evidence of the special structure of DNA was the observation that the amounts of adenine (A) and thymine (T) are almost equal in every type of DNA. The same applies to guanine (G) and cytosine (C). Investigations of synthetic DNA molecules have shown that DNA can adopt several different conformations. By far the most common form is B-DNA (right-handed helical rotation; the most stable structure for random sequence DNA molecule under physiological conditions; longer ant thinner).The A form (right handed; bases are not arranged at right angles to the axis of the helix, as in the B form; short and broad) and Z form (left handed; the DNA backbone takes on “zigzag“ appearance; elongated and slim) have been well characterized in crystal structure. In order to cope with the volume constraints it faces inside a cell or virus particle, DNA has to pack itself. In the appropriate solution conditions with the help of ions and other molecules, extended DNA chains collapse into compact, orderly particles that contain only a few molecules. This process is known as DNA condensation and it has been described in vitro and in vivo. The main potential medical application of DNA condesation is gene delivery in gene therapy.