Control of defect density is very important in all the processes of integrated circuits. Photomask plays a very important role in IC fabrication. Defect density has to be very well controlled in photomasks. It is not only important to work for a good process control in mask fabrication shop. It is also necessary that we understand various causes of these defects. There can be multifarious sources of these defects. Some defects are caused by improper processing, others by insufficent control in environment and still others by improper imaging. In this paper we present the results of a comprehensive study and classification of different defects found in photomasks. The sources of these defects are also mentioned. Some defects are not generally considered in present mask fabrication processes and in that respect, are new. This study has been performed in order to gear our mask making facility for VLSI where defect densities of the order of 1 defect per inch square might be needed. Information gathered in the course of this study can greatly enhance the capabilities of a mask fabrication shop for LSI.