Built between East and West Berlin.

People in Communist countries wanted to leave for political and economic reasons (Capitalist countries were much more attractive).

Great contrast between East and West Berlin.

West put investment into West Berlin to make East Berlin envious.

Increasing number of people moving from East to West and not coming back (mostly skilled workers that were important).

Barbed wire and then wall put up.

This tells us that:
Russia is worried about people in Communist section seeing how good it was in Capitalist section and worried about losing qualified people .

Showed West that Russia was worried about its control and popularity with people.