This special issue addresses open research topics related to ground observations of the Earth’s magnetic field. It consists of papers based on talks given during the XVIIth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing that was held in Dourbes (Belgium) from 4 September to 10 September 2016. Ground magnetic observatories are important for geomagnetism. They provide fixed, long-term, high-quality global time series, thus helping the scientific community to understand the geophysical processes of solid Earth as well as the interaction of the Sun with Earth’s magnetosphere and ionosphere. Data users such as space weather, space physics, and geomagnetic field modelling communities always require better quality data from observatories for their applications. This workshop aimed to improve observatory practice especially when faced with new challenges like advanced instrumentation development, higher sampling rates, faster definitive data processing, real-time delivery, and remote deployment. The event was successful in bringing together about 90 attendees from 37 different countries, of which 21 were European. The 72 presentations covered topics such as instrumentation, observation techniques and processing, data transmission, and applications. We invite the workshop participants and other researchers to submit their results to this inter-journal special issue of Annales Geophysicae (ANGEO) and Geoscientific Instrumentation, Methods and Data Systems (GI).