This chapter reviews the definition of the black-hole event horizon and outlines the proof that the area of its two-dimensional section cannot decrease. It shows why quantum mechanics requires black holes to have a temperature that is determined by their ‘surface gravity’ and entropy proportional to the area of the (two-dimensional section of the) event horizon. The chapter evaluates the number of perturbative microstates in string theory is evaluated and shown to be quite inadequate to explain the derived entropy of black holes. To get black holes from string theory, analogous solutions to the underlying classical field theory should be found. This is type II supergravity. The chapter provides a discussion on the (five-dimensional, extreme) black-hole solutions, with three charges, that have an event horizon with non-zero area.