A mechanistic understanding of biology requires appreciating spatiotemporal aspects of gene expression and its functional implications.Conditional expression allows for (ir)reversible switching of genes on or off,with the potential of spatial and/or temporal control.This provides a valuable complement to the more often used constitutive gene (in)activation through mutagenesis,providing tools to answer a wider array of research questions across biological disciplines.Spatial and/or temporal control are granted primarily by(combinations of) specific promoters,temperature regimens,compound addition,or illumination.The use of such genetic tool kits is particularly widespread in invertebrate animal models because they can be applied to study biological processes in short time frames and on large scales,using organisms amenable to easy genetic manipulation.Recent years witnessed an exciting expansion and optimization of such tools,of which we provide a comprehensive overview and discussion regarding their use in invertebrates.The mechanism,applicability,benefits,and drawbacks of each of the systems,as well as further developments to be expected in the foreseeable future,are highlighted.