This chapter highlights the pros of biological purification methods, such as the possibility to be carried out in situ at the contaminated site, cost efficiency, the ability to regenerate the sorbent, as well as an overall environment friendly character. Biosorption and bioaccumulation processes rely on natural metabolic processes when toxic ions are taken up instead of essential ions. The mechanisms that ensure biosorption, physical adsorbtion, ion exchange, formation of metal complex compounds and precipitation are analyzed further in this chapter. Data demonstrating the biosorbtion ability of cyanobacteria (Nostoc muscorum, Spirulina platensis and Aphanothece flocculosa) towards certain metal ions (Cu²⁺, Cd²⁺, Cr³⁺, Cr⁶⁺, Co²+, Ni²⁺ and Fe³⁺) are also detailed. As the bioaccumulation mechanisms reduction, chelation and precipitation are described. Cyanobacteria are ideal biosorbents and bioaccumulators because of their omnipresence in water and soil ecosystems and flexible metabolism. The chapter contains numerous examples of the use of cyanobacteria as detoxifying agents, with major emphasis placed upon two genera – Arthrospira (Spirulina) and Nostoc.