When a beam of X-rays strikes a crystal, the incident beam is split into many weaker beams having different directions and different intensities, thus giving rise to what’s called the diffraction pattern.  The nature of the diffraction pattern, which is to say the directions and intensities of the scattered X rays, is determined by the structure of the crystal (i.e. the arrangement of the atoms in the crystal).

On the other hand, if one knew the structure of the crystal, one could readily predict the nature of the diffraction pattern (i.e. the directions and intensities of the X-rays scattered by the crystal).

However, the problem facing the crystallographer is the converse: One observes the diffraction pattern - that is, one measures the directions and intensities of the X rays scattered by the crystal.  Can one then deduce the structure of the crystal (the atomic arrangement) giving rise to the observed diffraction pattern? 

The 1985 Nobel Prize in Chemistry was given for the solution of this problem.

The ability to solve crystal structures rapidly and routinely has important consequences.  Possibly the most important is the ability to relate crystal and molecular structures to biological activity. Thus, it becomes possible to understand life processes at the “molecular” level, to understand better how living things “work”, and to determine the cause of a disease. Then, it is much easier to devise better therapies and better drugs for the prevention and treatment of the disease with a minimum of adverse side effects.

In short, the ultimate result of learning more about the molecular structures of biologically active compounds is to improve human health.