PsbP is an extrinsic subunit of photosystem II (PSII) and optimizes the water- splitting reaction in higher plants in vivo. In addition to PsbP, recent genomic and proteomic studies have demonstrated the existence of many PsbP homologs in the chloroplast of higher plants; however, their molecular functions have not been investigated at all. In order to establish a unified view about the PsbP family in higher plants, PsbP homologs in a model plant Arabidopsis thaliana were classified into two PsbP-like (PPL) proteins and six PsbP domain (PPD) proteins by amino-acid sequence similarity. Their molecular functions, especially those of two PPLs, are discussed in terms of mRNA expression profiles in the public microarray database.