We use the the supersymmetric harmonic oscillator to introduce supersymmetry, and to motivate how world-sheet supersymmetry leads to space-time fermions. After a short review of Poincaré Lie superalgebras, we introduce the RNS-model, both as an example of a supersymmetric field theory and as a stepping stone to type-II superstrings. We motivate the GSO-projection through modular invariance and show how the massless spectra as well as the D-branes of type-II string theories arise. Type-I and heterotic strings are briefly introduced, and we conclude with an overview of the dualities that relate the five modular invariant supersymmetric string theories to one another, and to eleven-dimensional M-theory.