A substantial share of the electric energy is generated with synchronous generators that provide sustained alternating current (AC) voltage and frequency energy to regional and national power systems, which subsequently transport and distribute it to diverse users. In an attempt to reduce environmental effects, electric energy markets have recently become more open, resulting in more flexible distributed electric power systems. In such distributed systems, stability, quick and efficient delivery, and control of electric power require some degree of power electronics control to allow for lower power in the electric generators to tap the primary fuel energy potential better and increase efficiency and stability. This is how variable-speed electric generators (VSEG) recently came into play, up to the 400-megavolt ampere (MVA)/ unit size, and which have been at work since 1996. This paper provides coverage of variable-speed electric diesel generators (VSDEG) in distributed generation and their impacts on fuel efficiency and greenhouse gases (GHG). It discusses permanent-magnet-(PM) synchronous generators, solutions based on power electronics such as diesel-driven wound-rotor-induction generator, doubly-fed-induction generator (DFIG), rotating stator generator, and the application of continuously variable transmission to a VSEG. The benefits and limitations of the selected technologies are also presented. The list of references given at the end of the paper should offer aids for students and researchers working in this field.