The ideas of Technicolour or Dynamical Symmetry Breaking for the weak interactions are reviewed. The need for technicolour is established because of the gauge hierarchy problem. The obvious phenomenological consequences are explored with emphasis on the possible existence of light (less than 100 GeV) scalar particles. The extended technicolour or sideways generalizations of technicolour are discussed. The relations between technicolour and grand unified theories, groups breaking themselves and vacuum alignment are also explored. Finally there is a discussion of technicolour and the possible composite structure of quarks and leptons.