Heat shock protein gene expression is enhanced by proteotoxic stress, i.e., by conditions favoring protein unfolding. This upregulation of heat shock protein genes is mediated by heat shock transcription factor HSF1. A mechanism, the details of which are still elusive, senses adverse conditions and causes HSF1 to oligomerize and to acquire DNA-binding ability. The DNA-binding form of HSF1 then undergoes further conformational changes that render it transcriptionally competent. The current model in which heat shock protein 70 acts both as sensor of stress and as negative regulator of HSF1 oligomerization as well as alternative models involving additional protein factors are discussed.