We review the optical properties, fabrication, and applications of zero-mode waveguides (ZMWs) for single-molecule studies. These simple nano-structures allow individual molecules to be isolated for optical analysis at high concentrations. Fluorescent species are observed in a sufficiently small volume that the average number of fluorescent molecules is less than one, even at concentrations high enough for biochemical reactions to proceed at normal rates. Arrays of such structures can also be engineered into systems for real-time analysis of large numbers of single-molecule reactions or binding events. We also review the integration of ZMWs in different optical configurations and microfluidic systems.