To explain nanocrystalline materials, it should have been explained that “nano” concept at first. Nanostructures are relatively new and interesting subject to investigate, they are in a scale which is between 0-100 nanometers (1 nanometer, nm is equal to 10-9 meter). There are some types of nanostructured materials such as lamellar (1-dimensional), filamentary (2-dimensional) and crystallites (3-dimensional). If grains of the material are made up of crystals, then “nanocrystalline materials” term is what they are called. In this case, nanocrystalline materials have grains which are the size of < 100 nm typically. An example of nanocrystalline material of SnO2 is imaged with STM as seen in Fig. 1  and SEM image of nanocrystalline diamond thin films as seen in Fig. 2. 
Fig. 1. STM image of nanocrystalline SnO2 
Fig. 2. SEM image of nanocrystalline diamond thin films. 
In nanocrystalline materials there are numerous superior properties which actually are very useful and promising about future works and applications. The reason for the superior properties is that nanocrystalline materials have very high number of atoms at their grain boundaries which are in nanometer scale. Some of the properties that can be enhanced and increased by making nanocrystalline materials rather than the coarse-grained materials are higher strength, higher hardness, increased diffusivity, increased mechanical properties and so on . This is why people have studied in this subject for a very long time and since then very serious number of information are known about nanocrystalline materials and their properties. Of course, there are still many more things that people cannot learn or study yet, however, according to the results of current works, it can be said that very promising future about nanocrystalline materials is near.
Nanocrystalline materials have very huge content about synthesizing techniques, different kinds of properties, very variable and promising applications and etc. Thus, in this paper, it will be tried to be explained that some synthesizing techniques, their properties and possible future works and applications.
Nanocrystalline materials can be synthesized with top-down or bottom-up methods. In top-down method, the idea is based on starting from the bulk material and going down to the nano scale; and in bottom-up method the combination of small clusters is done.
Fig. 3. Some of the very common techniques for synthesizing NC materials. 
Different techniques are listed in Fig. 3.  In these synthesizing methods, one can control the grain size, morphology, texture and order of nanocrystalline materials by changing variables and processes. Hence, due to the fact that properties of NC materials are dependent on those; one can control the properties of those materials. Sol-gel, vapor deposition, chemical vapor deposition and gas condensation are...