摘要： Stimulated transitions in quantum systems represent the principal background for laser operation. Lasers play important roles in daily life and form a valuable part of photonics. Though the first lasers were supposed to have an impact on material processing owing to extreme energy concentration, currently the main applications of lasers are not based on high-energy abilities, but essentially on their compactness, cheapness, and efficiency. Optical communication, laser printers, and DVD players are the most common laser applications, and here semiconductor lasers are definitely unsurpassed. In this chapter we provide a brief introduction to the principles of laser operation and laser device engineering to emphasize where nanostructures can be useful and which components of lasers can be improved by means of nanophotonic solutions. Because of the scope of this book, a brief explanatory style is used which may confuse laser experts but at the same time may still appear complicated for newcomers. This chapter may appear complicated for readers not at all familiar with lasers, and in this case appropriate textbooks are suggested for further reading.