The transition from normal growth conditions to stressful conditions is accompanied by a robust upregulation of heat shock proteins, which dampen the cytotoxicity caused by misfolded and denatured proteins. The most prominent part of this transition occurs on the transcriptional level. In mammals, protein-damaging stress leads to the activation of heat shock factor 1 (HSF1), which binds to upstream regulatory sequences in the promoters of heat shock genes. The activation of HSF1 proceeds through a multi-step pathway, involving a monomer-to-trimer transition, nuclear accumulation and extensive posttranslational modifications. In addition to its established role as the main regulator of heat shock genes, new data link HSF 1 to developmental pathways. In this chapter, we examine the established stress-related functions and prospect the intriguing role of HSF 1 as a developmental coordinator.