Microneedles (MNs) are micron-scaled medical devices used to administer vaccines, drugs, and other therapeutic agents. The use of microneedles is known as microneedling. Microneedles are usually applied through even single needle^{[clarification needed]} or small arrays, called microneedle patch or microarray patch. The arrays used are a collection of microneedles, ranging from only a few microneedles to several hundred, attached to an applicator, sometimes a patch or other solid stamping device. The height of each needle ranges from 25μm to 2000μm. The arrays are applied to the skin of patients and are given time to allow for the effective administration of drugs.

While microneedles were initially explored for transdermal drug delivery applications, their use has been extended for the intraocular, vaginal, transungual, cardiac, vascular, gastrointestinal, and intracochlear delivery of drugs. Microneedles are also used in disease diagnosis, and collagen induction therapy. Although the concept of microneedling was first introduced in the 1970s, its popularity has surged due to its effectiveness in drug delivery and its cosmetic benefits.

Known for its minimally invasive and precise nature, microneedling is an easier method for physicians as microneedles require less training to apply and because they are not as hazardous as other needles, making the administration of drugs to patients safer and less painful while also avoiding some of the drawbacks of using other forms of drug delivery, such as risk of infection, production of hazardous waste, or cost.

Microneedles are constructed through various methods, usually involving photolithographic processes or micromolding. These methods involve etching microscopic structure into resin or silicon in order to cast microneedles. Microneedles are made from a variety of material ranging from silicon, titanium, stainless steel, and polymers. A variety of MNs types (solid, hollow, coated, hydrogel) has been developed to possess different functions. Some microneedles are made of a drug to be delivered to the body but are shaped into a needle so they will penetrate the skin. The microneedles range in size, shape, and function but are all used as an alternative to other delivery methods like the conventional hypodermic needle or other injection apparatus. Stimuli-responsive microneedles are advanced devices that respond to environmental triggers such as temperature, pH, or light to release therapeutic agents. The research on MNs has led to improvements in different aspects, including instruments and techniques, yet adverse events are possible in MNs users.

The concept of microneedles was first derived from the use of large hypodermic needles in the 1970s, but it only became prominent in the 1990s as microfabrication manufacturing technology developed. Later, the concept of MNs finally came into experimentation in 1994 when Orentreich discovered the insertion of tri-beveled needles to the skin could possibly stimulates the release of fibrous strand. The investigation on MNs' potential to improve transdermal drug delivery gradually raised public awareness of MNs. Since then, there has been massive research conducted on MNs, contributing to the development of different materials, types, and fabrication methods of MNs. Application and adverse events are explored. In the 2000s, clinical trials on MNs' use in drug delivery began.

Microneedles were first mentioned in a 1998 paper by the research group headed by Mark Prausnitz at the Georgia Institute of Technology that demonstrated that microneedles could penetrate the uppermost layer (stratum corneum) of the human skin and were therefore suitable for the transdermal delivery of therapeutic agents. Subsequent research into microneedle drug delivery has explored the medical and cosmetic applications of this technology through its design. This early paper sought to explore the possibility of using microneedles in the future for vaccination. Since then researchers have studied microneedle delivery of insulin, vaccines, anti-inflammatories, and other pharmaceuticals. In dermatology, microneedles are used for scarring treatment with skin rollers. As mentioned before, microneedles have also been explored for local targeted drug delivery at other drug delivery sites, such as the gastrointestinal, ocular, vascular etc., of which, ocular, vaginal and gastrointestinal have shown increasingnly convincing outcomes where they serve as a more efficient, localised drug delivery system, without the drawbacks of systemic exposure/toxicity.

The major goal of any microneedle design is to penetrate the skin's outermost layer, the stratum corneum (10-15μm). Microneedles are long enough to cross the stratum corneum but not so long that they stimulate nerves which are located deeper in the tissues and therefore cause little to no pain.

Research has shown that there is a limit on the type of drugs that can be delivered through intact skin. Only compounds with a relatively low molecular weight, like the common allergen nickel (130 Da), can penetrate the skin. Compounds that weigh more than 500 Da cannot penetrate the skin.