Integrating nanophotonics with electronics could enhance and/or enable opportunities in areas ranging from communications and computing to novel diagnostics(1,2). Light sources and detectors are important elements for integration(1), and key progress has been made using semiconducting nanowires(3-5) and carbon nanotubes to yield electrically driven sources(6-12) and photoconductor detectors(13-17). Detection with photoconductors has relatively poor sensitivity at the nanometre scale, and thus large amplification is required to detect low light levels and ultimately single photons with reasonable response time. Here, we report avalanche multiplication of the photocurrent in nanoscale p-n diodes consisting of crossed silicon-cadmium sulphide nanowires. Electrical transport and optical measurements demonstrate that the nanowire avalanche photodiodes (nanoAPDs) have ultrahigh sensitivity with detection limits of less than 100 photons, and subwavelength spatial resolution of at least 250 nm. Crossed nanowire arrays also show that nanoAPDs are reproducible and can be addressed independently without cross-talk. NanoAPDs and arrays could open new opportunities for ultradense integrated systems, sensing and imaging applications.