There are more than 30,000 species of spiders, most of which are venomous but unable to inflict serious bites; most spiders only cause minor effects. The effects of medically important spiders are sometimes underestimated; conversely, there is a misattribution of signs and symptoms to alleged spider bites. Diagnosis is rarely based on the identification of the spider; immunoassays detecting the spiders' venom are not available. Brown spiders (Loxesceles spp.) and widow spiders (Latrodectus spp.) may inflict life-threatening complications. Prompt recognition with subsequent appropriate treatment might prevent severe reactions and improve the prognosis.