Since their inception, optical detection and spectroscopy of single molecules have steadily expanded to an amazing variety of disciplines in natural sciences. Domains as varied as optical microscopy, quantum optics, nanophotonics, material science and soft-matter physical chemistry all have benefited from the new, average-free insights provided by the optical isolation of single molecules, quantum dots, metal nanoparticles, and other nanometre-sized objects. The techniques themselves have also made spectacular progress with the developments in super-resolution microscopy, time-resolved measurements, absorption-based detection, combination with mechanical or electrical manipulation and recording, live-cell imaging, and metal nanoparticle-enhanced phenomena. At the chemistry-biology interface, new probes are needed for the study of various biological processes, most of them in live cells or even live organisms, but also for superresolution microscopy. The plasmonics-chemistry interface includes studies of catalysis, diffusion in soft materials and nanofluidics. At the border between quantum optics, plasmonics and physical chemistry, low-temperature spectroscopy experiments provide candidates for the manipulation of single spins as qubits, while new structures can be designed as nanoantennas to enhance molecular fluorescence and a broad variety of nonlinear optical processes.