Two decades of research into functions of the ubiquitous transcription factor HIF have revealed pervasive roles in development, oxygen homeostasis, metabolism, cancer and responses to ischemia. Unsurprisingly, HIF activities impinge on many pathologies, for which underlying molecular mechanisms are actively sought. HIF is a member of the heterodimeric bHLH/PAS family of transcription factors, a set of proteins that commonly function in developmental pathways and adaptive responses to environmental or physiological stress. Similarities in the mechanisms that regulate gene targeting by these transcription factors create opportunities for extensive crosstalk between family members. Data supporting pathway interactions between HIF1a and other bHLH/PAS factors, both collaborative and antagonistic, is beginning to surface in the areas of cancer, circadian rhythm, and immune responses. This review summarises the status of HIF1a-bHLH/PAS protein crosstalk and is dedicated to the memory of Lorenz Poellinger, a pioneer investigator into the molecular mechanisms of HIF, AHR, and ARNT bHLH/PAS factors.