ABSTRACT There has been quite a number of informative papers published recently describing seawater-injection facilities for fields such as B.P.'s Forties (Ref. 1) and Chevron's Ninian (Ref. 2) in the North Sea and GUPCO's El Morgan in the Gulf of Suez (Ref. 3). Those waterfloods described were for relatively new developments, with reservoir permeabilities varying from 100-2,000 millidarcies. They are generally recognised as being successful floods, although each author notes various operating problems caused, for example by corrosion, scale formation and bacterial fouling. This paper attempts to provide a logical approach to identifying the essential causes of operating problems encountered in a waterflood. These problems can have a dramatic effect on the economics of oil production, particularly for older floods and with tighter formations, where permeabilities may be around 1‒10 millidarcies or less. Before discussing troubleshooting specifically, consideration is first given to the effects which lead to recognition that there is an operating problem. Waterfloods which use seawater tend to be most common, and have the highest flowrate. However, the following comments are just as appropriate to waterfloods using produced, aquifer or fresh surface waters. The problems may be classified into five groups