This is a repost from my other blog, first published on August 11, 2013. Since then, Aso has had some remarkable eruptions: check out the Global Volcanism Program link below for more details and images.
Monitoring (note: only the Japanese-language website seems to be working right now). See also the Japan Meteorological Agency – despite the name, they watch for all natural hazards in Japan. Currently JMA has no warnings posted for Naka-dake, which does have frequent, small-scale eruptions and is quite a tourist draw.
Webcam (in Japanese, but real-time images).
Featured image: ka.sha (busy & OFF). CC BY 2.0