What writing utensils would work for concrete walls? My boyfriend has concrete walls surrounding his at home office. He has expressed interest in using the walls as "story boarding" space for future mobile applications. I'm curious as to what writing utensil would be the very best option for clarity of penmanship on a concrete wall. Being able to be washed off after a period of time would be a bonus too.
When dealing with a concrete surface, the first consideration is the smoothness of the surface. If that is acceptable, and of reasonably decent flatness, your options expand nicely. If it's a coarse surface with lots of pits, cracks and other irregularities, chalk may be the only answer. It's messy and washable, though.

If the surface can be painted, consider to use a dry-erase paint product. I found many with a quick search: 

Benjamin Moore Dry Erase Paint

Such products will enable one to use standard office-type dry-erase markers of multiple colors and tip sizes, with easy dry-erase erasers used to remove the work.

Depending on the scope of the use (area of walls to be painted), the biggest disadvantage I see is that the room would be very white.