The single electron effect is a consequence of reduced capacitance in confined islands when dimensions are reduced, usually to the sub-30 nm range. Small capacitances, of the order of aF’s, result in a measurable discreteness in the transfer of electrons through the islands because of the large electrostatic energy needed for transfer of charge. In semiconductors, single electron effects occur together with strong quantum-confinement effects due to the smaller density of states. In a single electron transistor, the discrete transfer of the charge is modulated by a gate voltage, and circuits analogous to CMOS can be fabricated. In most single electron memories, the effect of the single electron charge influences transport in a field-effect channel through screening, i.e., discreteness effects are coupled to the traditional field-effect of the transistor. While powerful demonstrations of room temperature operation of single electron transistors, single charge transfer devices, and simple gates with gain have been made, the use of the devices in general purpose electronics is limited by large impedance, low currents, and fluctuation effects. One particularly unique use of single electron transistor has been in charge profiling due to the strong intrinsic charge sensitivity. Memories based on single electron effects, however, are finding wider appeal because of large improvement in power, speed, voltage, and reliability characteristics over traditional non-volatile memory alternatives and their strong compatibility with present-day practice of silicon microelectronics. Such memories have been demonstrated at large dimensions (100’s of nm) where numerous discrete nanocrystal islands are employed as well as in the ultimate limits of field-effect when device dimensions reduce to nearly 10 nm in dimension. We will discuss the properties of the single electron device structures and relate them to the underlying physics.