The development of electrochemical sensors and biosensors by integration of transducers and recognition elements has received continuously growing attention and interest, by virtue of the extraordinary features, such as the sensitivity, simplicity, practicality, portability, ease of operation and even low production cost. Taking advantage of the remarkable advances in nanotechnology, the sensitivity, selectivity and accuracy are increasingly being improved. This chapter presents a clear and concise conceptualisation and classification of sensors, and more specifically of electrochemical sensors and biosensors, highlighting the impact of nanomaterials on the development of such devices. Finally, an interesting case study is presented on the use of an advanced gold nanoparticle on a glassy carbon electrode for electrochemical sensing of bisphenol A with low detection limits.