Ion channels are proteins that form nanometer-scale pores in cell membranes. Some provide the molecular basis of action for nerve and muscle, while others (e.g., bacterial pore-forming toxins) cause deleterious effects in mammalian cells and tissue. We have been developing the latter class of nanopores for the detection, characterization, quantitation, and identification of single molecules. We will discuss the potential use and limits of this method for practical applications, including DNA sequencing, sizing individual molecules, measuring the forces between single molecules, therapeutics development, and the identification of synthetic nanoparticles, proteins, and other biomarkers.