Bioelectromagnetism covers a number of sub-disciplines, but is firstly concerned with the ability of living systems to generate electrical currents and magnetic fields. Secondly, it covers the susceptibility of tissue to be stimulated by these currents or fields, and the ability of some organisms to sense low level fields. Thirdly, it also covers the possible health implications of exposure to low level environmental fields, including those associated with the transmission and distribution of electric power and of telecommunication systems. Fourthly, it presents the various therapeutic uses of these fields. Some of the effects are well-established, whereas others remain controversial. This chapter discusses the nature of this uncertainty.