[en] This article reviews mechanisms of interaction between extremely low frequency (ELF) fields and biological systems. The emphasis is on simple quantitative models, supplemented by results of more detailed computer analysis. Scaling relations are reviewed which underlie the coupling of external fields to the inside of the body, and of extracellular fields to potentials induced in cell membranes. The relation between time domain and frequency thresholds for stimulating excitable tissues is reviewed, based on the work of Reilly. A range of thermal and non-thermal interaction mechanisms are reviewed, with respect to the question of possible effects of weak ELF fields in biological systems. Each mechanism is characterised by a threshold field strength (below which no observable response is produced) and dynamics; in each case, the anticipated < I>in situ tissue strengths for producing observable effects are far higher than the range of 'weak' fields considered in this review. However, the bioeffects literature is unclear in many places, with numerous speculations about possible mechanisms for weak field effects and many reports of effects that are not clearly interpretable in terms of the mechanisms discussed in this article. (author)