Traditionally, the answer has been 'massive bloodshed,destruction and large-scale robbery of mainly Church posessions".But we now know that these stories - almost always written by monks- were in most cases vastly exaggerated. The monks no doubtresented the fact that silver an gold Church posessions, easy asthey usually were to carry away, were indeed often taken - and ofcourse the Normans were heathens to boot, which also did not endearthem to the monks who wrote about them. But of the towns reportedlyburnt to the ground with all their inhabitants cruelly murdered weread in other sources that festive annual fairs were held there acouple of months later.The Viking (or rather, "Norman" as they were always called atthe time) invasions took the form of settlement in regions likeeastern England, Ireland and Normandy (hence the name). They alsosettled in Sicily which they even came to rule with the blessing ofthe Pope and in Byzantium and southern Russia around the Black Sea.What did they 'cause' there? Mostly economic growth, protection ofthe Pope against several enemies, and a big upsurge in the tradefrom and to what is now called Russia.