The domain Archaea comprises diverse life forms distinguished by their unique evolutionary history, cellular structures, and metabolic pathways. Classified initially as “extremophiles,” archaea are now well known to inhabit a wide range of moderate environments, from the human microbiome to terrestrial and marine habitats. Archaea’s ability to thrive in moderate and extreme conditions, such as high salinity, high temperatures, and anaerobic environments, has made them invaluable for studying fundamental biological processes. Advances in archaeal genetics and laboratory cultivation have allowed research to establish a genetic system for key archaeal model organisms belonging to four major groups: methanogens, halophiles, thermophilic euryarchaea, and crenarchaea. These organisms employ distinct metabolic strategies, including methane production, extreme osmoregulation, thermophilic growth, and acidophilic adaptations. Methanogens are the only group on earth capable of producing methane and play a crucial role in the global carbon cycle. Halophiles provide information on adaptation at high salinity and are also used to study DNA replication and repair after UV exposure. Thermophilic model organisms shed light on adaptation to higher temperatures, and acidophilic archaea, like those in the order Sulfolobales, are important models for researching protein stability, stress response mechanisms, and bioenergetics under harsh conditions. More recently, researchers have cultivated other archaea members, ammonia-oxidizers, shedding light on their involvement in global nitrogen cycling. This chapter will explore key archaeal model species from these groups, highlighting their physiological adaptations, genetic tractability, and significance in shaping our understanding of microbial life.