The development of miniaturized electrospray ionization sources (known as microspray and nanospray) has revolutionized biological mass spectrometry because these sources have low sample consumption requirements, utilize low flow rates (nL/min), and exhibit improved ion transfer efficiency from source to detector versus the conventional electrospray ionization source. Here, we review the development, advantages, and applications of miniaturized electrospray sources. We also highlight some of our own research aimed at producing high-durability nanospray emitters for utility in coupling capillary separations techniques with miniaturized electrospray sources.