Summary form only given.Over the 1980s/1990s, external cavity diode lasers have been successfully utilized for, among others, linewidth narrowing, suppression of unwanted secondary longitudinal modes, and reduction of modulation-induced frequency chirp. Although many questions have been answered and our understanding of the underlying mechanism as to the sensitivity of the diode laser subject to external optical feedback has significantly increased, many questions still remain, the solution of which are essential for control of the stability of these lasers.