Protists are a diverse, polyphyletic group of eukaryotic organisms that do not form a single formal taxon. They can be unicellular or multicellular and vary widely in their morphology, reproductive strategies, nutritional acquisition, and methods of locomotion, which may involve contractile vacuoles, cilia, flagella, pellicles, or pseudopodia, though some lack organelles like mitochondria. Their taxonomy is rapidly changing as relationships are reassessed using newer techniques, and they include significant pathogens and parasites.