In this review, phenomena and ideas connected with strongly interacting electrons in condensed matter systems will be outlined. A simple picture which regards electrons as basically a free gas of quantum particles is the basis of our understanding of all electronic behaviour of metals and insulators. In the last few decades, families of systems have been explored in which the interaction energy is much larger than the energy of free motion, or kinetic energy. These include transition metal oxides with unfilled d electron shells, rare earth intermetallics with f electrons, etc. Their behaviour is rich and not well understood in terms of existing paradigms. I point out some experimental features common to these, e.g. that they are ‘bad’ metals with unusually low quantum coherence temperatures. I also briefly give illustrative examples such as cuprates (which are high temperature superconductors), man-ganites (exhibiting colossal magnetoresistance) and heavy fermions (rare earth intermetallics). Some of the ideas, theories and methods available for describing these sysytems are outlined. It is suggested that a new general approach may be needed for this qualitatively different regime of many electron behaviour
