The science of getting the right amount of a drug to the right part of the body at the right time. This may be done by controlled release microchips, self-assembled biomolecule vectors, or ceramic capsules that degrade with time.
Nanotechnology
This is a particularly hot topic in medicine right now, especially for gene therapy. Currently, the accepted method for getting new genetic material into of someone's cells is by using a crippled virus to carry it in. This is considered dangerous by some and has proven fatal on at least one occasion.
The main candidates for bearing drugs to extremely specific locations within the body are lipids and proteins. Molecules from both of these broad classes can be made to self assemble into large container-like structures capable of carrying their payloads from the bloodstream, digestive tract, or muscle tissue into cells. These tiny delivery vehicles are called vectors.
Some self-assembled drug delivery vectors take advantage of the difference in pH between the inside and outside of cells. The vectors remain stable outside of the cells, but immediately burst in response to the low pH within.
Vectors may not only be used for gene therapy, but also for "smart drug delivery", in which medicine is taken orally or injected into the body but it is only released from the delivery vectors when they come into contact with cells or tissue of the right type. For example, a cancer drug may only be released when it comes into contact with cancerous cells or a drug that is toxic to the liver may only be released in the kidneys, where it is needed.
Blood cells have also been taken over and used as smart ddrug delivery vectors.
Biochemistry
In general, only "left handed" amino acids are found in nature. Peptide drugs can be made from "right handed" amino acids so that they are not broken down by the digestive system or stopped by the immune system. This has proven to work for at least one HIV drug.
Some drugs are designed so that they only affect cells with specific receptor sites. This allows them to only affect the cells that they should be working on.
Microelectronics
Controlled release microchips may eventually be able to release small doses of potent medicines immediately when a group of sensors determines that it is necessary. For example, a controlled release microchip in an elderly person may release an anticoagulant when sensors implanted in other parts of their body show concurrent signs that they are in the early stages of a stroke.
Ceramics
Ceramic materials may be used to release small amounts of a potent drug steadily over a long period of time.
Machines
Machines for drug delivery include the insulin pump invented by segway inventor Dean Kamen.
"Delivering the Goods"
A Science News Article
http://www.sciencenews.org/20030118/bob10.asp
"Futuretech:A Pill with Your Name on It"
A Discover
Magazine Article
http://www.discover.com/dec_01/feattech.html
Blood Cells Deliver Drugs Safely
A New Scientist Article
http://www.newscientist.com/news/news.jsp?id=ns99992295
Ultrasound used to target drug delivery in the brain
A New Scientist Article
http://www.newscientist.com/news/news.jsp?id=ns99991644