Many sci-fi films have been made about miniaturization of humans. Good examples are the "Honey I Shrunk The Kids/Baby/Ourselves", "Innerspace", and "Fantastic Voyage", where the main characters are miniaturized via a beam of some sort. For a long time people have aspired to shrink themselves down and explore the miniature world without a microscope.
These films oversimplify the complicated issues involved with miniaturization and completely fail to give any kind of explanation for the process the characters used. In reality, you cannot simply point a beam of energy at something and miniaturize it. The problem is very complicated.
The most glaring issue is that atoms, the particles that compose the world and everything in it, do not scale. They are essentially a fixed size and nothing is going to change that fact. There is no way to reduce the size of a proton, neutron, or electron, the basic building blocks of atoms. You also cannot change the number of sub-atomic particles in an atom without dramatically changing its chemical characteristics.
If we can't reduce the size of atoms in a human being or change their sub-atomic composition, what can we do to miniaturize a human? The only other remote possibility is reducing the number of atoms that compose the structures in the body. The problem with this is that it would be a monumental task to figure out which atoms could be extracted without destroying vital organs and dramatically altering the behavior of the cells.
Even if it were possible, there is still a finite limit to how small the human could become before so many atoms were removed that it altered the body's function. For example, it is very hard to believe that reducing the human brain's size to the size of an ant's by cutting out atoms would not impact the human's mental ability. A structurally-complex organ like the human brain simply cannot fit in an ant or bacterium, it is impossible. Even if it were possible, how would you extract the atoms from inside the body in the first place?
In addition to the structural problem, cutting atoms out of the proteins and amino acids in the body would totally change their function. Cutting out atoms in DNA would be like causing cancer mutations all over the body. These structures operate completely based on their atomic composition and changing them would have HUGE impacts on how the body operated at the cellular level.
Even if we cannot shrink living structures, can we still shrink non-living structures? This does appear plausible, as demonstrated by the miniature robots and structures created using STM microscopes. Miniature statues of bulls, cars that fit on the head of a pin, and other amazing things have been made and much more is possible since these objects have no internal complexity. Still, it is not clear how a normal size object could be quickly reduced in size to microscopic proportions since the problem of "how do we remove atoms or molecules" reappears.
In summary, it is clear that miniaturization of complex living things appears implausible, if not impossible. Atoms cannot be scaled and the human body is simply too complicated in structural and chemical terms to simplify it without dramatic impacts. However, it would be possible to shrink simple structures with no internal complexity, if the technology were developed to discard excess atoms or molecules and replicate the original structure.