Photosynthetic eukaryotes encode two copies of the CSP41 (Chloroplast Stemloop binding Protein of 41 kDa) protein that are of cyanobacterial origin. In Arabidopsis thaliana, the two CSP41 proteins belong to the group of most-abundant chloroplast proteins. Multiple functions have been described for CSP41 proteins, including roles in chloroplast rRNA metabolism and transcription. CSP41a and CSP41b interact physically. Recent data show that CSP41b is an essential and major component of high-molecular weight complexes that form in the dark, disassemble in the light, and bind chloroplast mRNAs coding for photosynthetic proteins and some ribosomal RNAs, but not the plastid-encoded RNA polymerase (PEP). This, together with the effects seen in leaves of plants lacking CSP41b, implies that complexes containing CSP41 proteins stabilize untranslated mRNAs and precursor rRNAs. This occurs in a redox-dependent manner and seems to be important in the absence light when the translation is less active. In this scenario, translation and transcription is secondarily affected by the decreased transcript stability.