Halophiles inhabit environments from moderate salinity to saturation, and the most hypersaline systems are chiefly populated by haloarchaea (class Halobacteria). This chapter synthesizes their physiology, diversity, and industrial potential. Most haloarchaea grow at mesophilic temperatures and near-neutral pH, but the lineage includes alkaliphiles, acidophiles, psychrotolerant strains, and magnesium-requiring specialists. Hallmarks include ether lipid membranes, S-layers or polysaccharide walls, and intracellular KCl that stabilizes proteins. Haloarchaea are isolated from salt lakes, salterns, halite deposits, commercial salts, and increasingly marine settings. Recent taxonomic revisions expanded orders and families and recognized nano-haloarchaea and halophilic methanogens. Functionally, haloarchaea furnish robust biocatalysts and cellular chassis for high-salt operations: proteases, lipases/esterases, and amylases retain activity at molar salinity and in organic solvents, enabling leather processing, cleaning, lipid conversions, and starch hydrolysis. Several genera accumulate PHAs; Haloferax mediterranei produces PHBV from diverse feedstocks, with seawater media and hypoosmotic lysis simplifying recovery. In bioremediation, haloarchaea degrade hydrocarbons, decolorize azo dyes at up to 5 M NaCl, and mediate metal detoxification via transport, transformation, and extracellular polymers. High salinity suppresses contamination and can obviate desalination pretreatment. We outline opportunities in enzyme engineering, strain domestication, and process intensification and challenges in continuous operation and long-term stability.