The objective of this paper is to present an overview of topics related to one of the fundamental parameters for semiconductor lasers - the linewidth broadening factor alpha that describes the coupling between carrier-concentration-induced variations of real and imaginary parts of susceptibility. After introducing the definition of alpha and discussing its dependence on carrier concentration, photon energy, and temperature, a short historical summary is given on how the concept of alpha evolved over the past two decades. This is followed by a discussion of alpha dependence on device structure in gain-guided and subdimensional lasers (quantum wells and quantum wires). The bulk of the paper is devoted to a detailed review of laser properties influenced by alpha and of associated methods of estimating the value of alpha. Results of measurements reported to date are collected and the most reliable methods are indicated.