Atomic layer deposition (ALD) is a thin film deposition procedure where growth occurs in a layer-by-layer fashion. Growth of a binary material AB occurs first by passing a gaseous stream of a precursor containing element A over the substrate, to afford a monolayer of the precursor on the surface. Then, an inert gas purge is carried out to remove excess precursor and any reaction products. Next, a gaseous stream of a precursor containing element B is passed over the substrate. This precursor reacts with the monolayer on the surface containing element A to afford the desired material AB. Finally, a second inert gas purge is conducted to remove any excess precursor and reaction products. These four steps constitute a growth cycle. A key feature of ALD is its self-limiting nature, in which the precursor containing element A reacts with surface reactive sites until they are all consumed. At this point, growth ceases, to afford a monolayer of the precursor on the substrate surface. Self-limiting growth gives excellent conformal coverage of high aspect ratio features because the growth is controlled by surface reactive sites, and also allows sub-nanometer control of film thicknesses by adjusting the number of growth cycles. The success of ALD is directly related to the chemical precursors that are used to deliver the desired elements for a given material. ALD precursors must combine good volatility, thermal stability at the deposition temperature, as well as high reactivity toward a second precursor to afford the desired material. The reaction products must be volatile so that they are not incorporated into the growing film. Moreover, chemical precursors must have chemical reaction paths built into them so that the desired materials are obtained. These properties are difficult to incorporate into molecules, and lack of appropriate precursors represents a major bottleneck for the wider applications of ALD. This tutorial will review ALD precursors that have been reported for a variety of materials. Emphasis will be focused on obtaining volatility, high thermal stability, and high reactivity toward a second precursor. Examples will be chosen where chemical reactivity is designed into the precursors to obtain desired materials with high purities. Difficult current challenges in ALD include design of precursors for the growth of electropositive element films, integration of appropriate ALD precursor chemistry into manufacturing processes, and the possibility of precursor design to achieve inherent area-selective deposition. Additional future challenges related to ALD precursors and chemistry encompass the emerging atomic layer etching (ALE) and atomic layer clean (ALC) techniques. The importance of precursors and chemistry in future ALD, ALE, and ALC processes will be highlighted.