The adaptive responses of the photosynthetic apparatus of Rhodophyta to light quality and quantity, relative to their light-harvesting complexes, the phycobilisomes (PBS) and the PBS components, particularly R-phycoerythrin, are reviewed in relation to pigment content, photosynthetic performance, algal growth and morphology in both the short and long term. Information available on fine structure, pigment composition and adaptive response of hemidiscoidal phycobilisomes from Cyanophyta is presented and critically compared to that, still scarce, regarding the hemiellipsoidal phycobilisomes, which are widespread in Florideophyceae containing R-phycoerythrin. Molecular characteristics and spectroscopic variations of this pigment are considered in relation to different light conditions. Particular emphasis is placed on the presence of well structured phycobilisomes during maximum photosynthetic activity, which appears to be not so strictly related to pigment content, and on the spectral changes of R-phycoerythrin. as a response to different wavelengths and irradiances of light.