The World of Algae -- Cyanobacterial Diversity in Relation to the Environment -- Toxicity of Fresh Water Algal Toxins to Humans and Animals -- Toxicity of Sea Algal Toxins to Humans and Animals -- The Role of Allelopathy for Harmful Algae Bloom Formation -- Checklist of Phytoplankton on the South Coast of Murcia (SE Spain, SW Mediterranean Sea) -- Toxic Pseudo-nitzschia Populations from the Middle Tyrrhenian Sea (Mediterranean Sea, Italy) -- Algal Blooms in Estonian Small Lakes -- Comparative Estimation of Sensor Organisms Sensitivity for Determination of Water Toxicity -- Biochemical Method for Quantitative Estimation of Cyanobacteria Toxins -- Using of Laser-Doppler Spectrometry for Determination of Toxicity Degree of Chemical and Natural Compounds -- PCR Techniques as Diagnostic Tools for the Identification and Enumeration of Toxic Marine Phytoplankton Species -- Detection of Phytoplankton with Nucleic Acid Sensors -- Development of Sensors to Trace Toxins from Dinoflagellates and Other Algae to Seafood -- Recombinant Formaldehyde Dehydrogenase and Gene-Engineered Methylotrophic Yeasts as Bioanalitycal Instruments for Assay of Toxic Formaldehyde -- Electrochemical Sensing Methods: A Brief Review -- Oddities and Curiosities in the Algal World. This volume contains the lectures and seminars given at the NATO Advanced Study Institute on "Sensor Systems for Biological Threads: The Algal Toxins Case", held in Pisa, Italy in October, 2007. The Institute was sponsored and funded by the Scientific Affairs Division of NATO. It is my pleasant duty to thank this institution. This ASI offered updated information on how far the research on algal toxins has gone in the exploration of structures, biosynthesis and regulation of toxins, and the development of technology for bio-monitoring these c- pounds. Algae can form heavy growths in ponds, lakes, reservoirs and sl- moving rivers throughout the world; algae can house toxins which are - ually released into water when the cells rupture or die. Hundreds of toxins have been identified so far. Detection methods, including rapid screening, have been developed to help us learning more about them, especially to find out which toxins are a real threat for people and what conditions encourage their production and accumulation. Early detection of algal toxins is an - portant aspect for public safety and natural environment, and significant efforts are underway to develop effective and reliable tools that can be used for this purpose.