Molecular nanotechnology describes engineered nanosystems (nanoscale machines) operating on the molecular scale. It is especially associated with the molecular assembler, a machine that can produce a desired structure or device atom-by-atom using the principles of mechanosynthesis- mechanically guided chemical synthesis- is fundamental to molecular manufacturing. It is a branch of engineering that deals with the design and manufacture of extremely small devices, that is, nanosystems or devices, built at the molecular level of matter. The proposed application of Molecular Nanotechnology is the ability to design and engineering material at nanoscale level, encompassing a wide variety of possible commercial applications. The projected applications of Molecular Nanotechnology include: Smart materials and Nanosensors (material engineered and designed at nanometer scale for a specific task), Replicating nanorobots, Medical nanorobots and Phased-array optics. Nanotechnology will replace our entire manufacturing base with a new, radically more precise, radically less expensive, and radically more flexible way of making products. The potential social impacts of Molecular Nanotechnology include: The maintainance of historical trends in manufacturing right up to the fundamental limits imposed by physical law, producing remarkably powerful molecular computers. Although have potential benefits, the technology concerns risks, having daunting risks, especially some analysts believe that this technology could lead to a Technological Singularity. The other risk highlights Self-replicating machines. Molecular nanotechnology might permit weapons of mass destruction that could self-replicate. Grey goo, a hypothetical end-of-the-world scenario involving molecular nanotechnology in which out-of-control self-replicating robots consume all matter on Earth while building more of themselves, a scenario that has been called ecophagy (that means “eating the environment").