Wigner's theorem is a cornerstone in the mathematical foundations of Quantum Mechanics. It states that if a bijective map on the set of all pure states preserves the transition probability, then there exists either a unitary or an antiunitary operator which induces this map in a natural way. In my talk I will present an elementary proof of this famous result and explain its connection to Quantum Mechanics. Then I will present some of its most recent generalisations.