Spider envenomation, generically known as arachnidism, is described in many places around the world. In terms of medical importance, the following genera of animals stand out as the main origin of the morbid condition: Atrax (Sidney funnel web spider), Steatoda (false black widow), Latrodectus (black widow), Loxosceles (brown spider) and Phoneutria (armed spider), the last three causing accidents involving spiders in Brazil. This article, part 1 of 2, aims to present the main aspects of arachnidism in the country, with an emphasis on the biology and geographical distribution of spiders, biochemistry of the venom, pathogenesis and epidemiology of arachnidism, as well as prevention of the morbid condition. KEY WORDS: Arachnidism; araneae; spider bites; spider venoms.Spider envenomation, generically known as arachnidism, is described in many places around the world. In terms of medical importance, the following genera of animals stand out as the main origin of the morbid condition: Atrax (Sidney funnel web spider), Steatoda (false black widow), Latrodectus (black widow), Loxosceles (brown spider) and Phoneutria (armed spider), the last three causing accidents involving spiders in Brazil. This article, part 1 of 2, aims to present the main aspects of arachnidism in the country, with an emphasis on the biology and geographical distribution of spiders, biochemistry of the venom, pathogenesis and epidemiology of arachnidism, as well as prevention of the morbid condition. KEY WORDS: Arachnidism; araneae; spider bites; spider venoms.