Ferredoxin-NADP+ (oxido)reductase (EC 1.18.1.2, FNR) is an FAD-containing enzyme that catalyzes the reversible electron transfer between NADP(H) and electron carrier proteins such as ferredoxin and flavodoxin. Isoforms of this flavoprotein are present in chloroplasts, mitochondria, and bacteria in which they participate in a wide variety of redox metabolic pathways. Although ferredoxin-NADP+ reductases have been thoroughly investigated and their properties reviewed on several occasions, considerable advances in the understanding of these flavoenzymes have occurred in the last few years, including the characterization of cDNA and genomic clones encoding FNR proteins from plants, algae, vertebrates, and bacteria, determination of the atomic structure of a plant FNR at high resolution, and the expression of functional reductases in microorganisms like Escherichia coli and Saccharomyces cerevisiae. The aim of this article is to summarize information gained through these recent developments, including the phylogenetic relationships among ferredoxin reductases and the key structural features of the plant FNR family. Other aspects such as the catalytic mechanism of FNR and the molecular events underlying biogenesis, intracellular sorting, folding, and holoenzyme assembly of this important flavoenzyme are also discussed in some detail. Ferredoxin-NADP+ reductases display several outstanding properties that make them excellent model proteins to address broad biological questions.