The genus Haageocereus (Cactaceae) is almost endemic to Peru, occupying arid areas of river valleys draining westwards off the Andes. Extensive fieldwork, morphological studies, literature revision and a study of the types led to the delimitation of 9 species and 6 heterotypic subspecies. Nomenclatural innovations include H. acranthus subsp. backebergii N. Calderón subsp. nov., H. versicolor subsp. pseudoversicolor (Rauh & Backeb.) N. Calderón comb. nov. and the recognition of H. repens Rauh & Backeb. Descriptions and keys are accompanied by line drawings and plates including SEM photos of the seeds for almost every taxon. Exsiccata prepared during this study constitute approximately 80% of existing Haageocereus herbarium samples from wild origin. Conservation assessments based on the IUCN Red List categories (2001) determined 3 taxa as Critically Endangered (CR), 8 taxa as Endangered (EN) and 5 taxa as Vulnerable (VU) in Peru.