With the growing interest in quantum information science since the beginning of the 21st century, research in different kinds of single-photon sources was growing. Early single-photon sources such as heralded photon sources that were first reported in 1985 are based on non-deterministic processes. Quantum dot single-photon sources are on-demand. A single-photon source based on a quantum dot in a microdisk structure was reported in 2000. Sources were subsequently embedded in different structures such as photonic crystals or micropillars. Adding distributed bragg reflectors (DBRs) allowed emission in a well-defined direction and increased emission efficiency. Most quantum dot single-photon sources need to work at cryogenic temperatures, which is still a technical challenge. The other challenge is to realize high-quality quantum dot single-photon sources at telecom wavelength for fiber telecommunication application. The first report on Purcell-enhanced single-photon emission of a telecom-wavelength quantum dot in a two-dimensional photonic crystal cavity with a quality factor of 2,000 shows the enhancements of the emission rate and the intensity by five and six folds, respectively.