Single layer semiconductor transition metal dichalcogenides (TMDs) possess outstanding properties that are desirable for development of 2D nanoelectronics and optoelectronics. One of outstanding properties is large exciton and trion binding energy rendering them to be stable even at room temperature, which governed nearly all optical characteristics observed in monolayer TMDs. However, behaviors of excitons and trions are very sensitive to external factors, including strain, dielectric of surrounding environment, electrical and chemical doping, and temperature. These features provide various approaches to manipulate in controllable manner optical and electronic properties of monolayer TMDs and their in-depth understanding is essential to exploit them for future optoelectronic devices. Here we review basic exciton properties and the impacts of charged doping, strain, dielectric screening, temperature and excitation intensity on excitonic behaviors reflected via photoluminescence spectra for monolayer of TMDs. (C) 2016 Elsevier B.V. All rights reserved.