Marine biotoxins, mainly produced by microalgae, show a wide range of structural diversity from simple amino acids to some of the largest and most complex natural products identified to date. Moreover, they display remarkable biological activities, and some compounds are even considered the most potent nonproteogenic venoms. This chapter presents the structures, analogues and structureâactivity relationships of most marine natural products biosynthesized by microalgae with toxic properties towards humans and/or marine organisms. The chapter includes excitatory amino acids (domoic acid and derivates, kainic acid and β-N-methylamino-alanine), alkaloids (saxitoxins and tetrodotoxins) and four main groups of polyketides: linear and macrocycled polyethers (okadaic acid and derivates, pectenotoxins, azaspiracids and goniodomins), polyether ladders (yessotoxins, brevetoxins, brevisulcenals, brevisulcatic acids, gymnocins, ciguatoxins, maitotoxins, gambieric acids, gambierol, gambieroxide, gambierone and prymesins), cyclic imines (spirolides, gymnodimines, pinnatoxins and pteriatoxins, prorocentrolides, spiro-prorocentrimine, symbioimine and portimine) and long carbon-chain polyols (palytoxins, ostreocins, mascarenotoxins, ovatoxins, ostreol, amphidinols and karlotoxins). © 2017 Elsevier B.V.