In this review, I have discussed a number of issues related to detailed Hi imaging of the outskirts of nearby spiral and irregular galaxies done with current interferometers, such as the WSRT, the VLA, the ATCA and the GMRT. I have been selective in my topics, and avoided to discuss the issue of star formation in extended Hi envelopes, since this is covered elsewhere in this volume. Most topics are related to the dark matter problem in one way or another. A number of results have been obtained by making case studies with the emphasis on improved sensitivity, rather than by simply observing more objects from a list.
The Square Kilometer Array project is advancing at great strides, and the associated approach of setting a challenging imaging target requiring the development of new instrumentation to reach it is soon going to bear fruit. Indeed, extensive Hi imaging surveys of relatively nearby galaxies will start next year with SKA pathfinders, such as the WSRT Apertif survey, and the SKA precursors ASKAP (the WALLABY survey) and MeerKAT (the deep, targeted MHONGOOSE survey, and also the MALS survey). Some of the projects for the first phase of the SKA telescope, SKA1, are described in the SKA science book (e.g., Blyth et al. 2015; de Blok et al. 2015). All these new surveys will bring fresh data of high quality, which can be brought to bear on the scientific problems discussed above and on other, related subjects, and will most likely lead to new insights and discoveries.
Acknowledgements I thank Lia Athanassoula for fruitful discussions and a careful reading of the manuscript. I also thank the editors for inviting me to write this review, and in particular Johan Knapen for his careful editing. I thank David Malin for sending me the deep image of M83 with the shallower image superimposed, and David Martínez-Delgado and Taylor Chonis for providing the image in the lower right panel of Fig. 5. Erwin de Blok supplied the script and Se-Heon Oh and Andrea Macciò supplied data for the construction of Fig. 12. Deidre Hunter helped out with Fig. 15. Ed Elson supplied the Hi data on NGC 2915 from which I could make the top panel of Fig. 16. Jing Wang clarified an issue with the Bluedisk sample.
I acknowledge financial support from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313 to the DAGAL network, from the CNES (Centre National d'Etudes Spatiales - France), and from the “Programme National de Cosmologie et Galaxies” (PNCG) of CNRS/INSU, France. This research has made use of NASA's Astrophysics Data System. Use was made of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration to produce parts of Fig. 5, Fig. 7, Fig. 11 and Fig. 16.