Book excerpt: Graphene-based nanocomposites are very useful in detecting toxic chemicals such as heavy metals, inorganic anions, phenolic compounds, pesticides, and chemical warfare agents. The book presents recent progress on relevant topics: Toxicity of chemicals, importance of electrochemical sensors, different types of graphene-based nanomaterials, Neurotoxins and electroanalytical detection of toxic chemicals. Keywords: Graphene-based Nanocomposites, Electrochemical Sensors, Toxic Chemicals, Sensors for Toxic Molecules, Graphene-Metal Oxides, Graphene-Metal Chalcogenides, Graphene-Polymer Nanocomposites, Graphene-Carbon Nanotubes, Graphene-Carbon Nitrides, Graphene-MOF Composites, Heavy Metals, Phenolic Compounds, Pesticides, Chemical Warfare Agents.