Optical mapping techniques allow the recording of simultaneous voltage and calcium variations characterising the action potential as well as calcium transients with high spatial resolution. Dual voltage and calcium recording is used to characterise transmembrane potentials and calcium variations in the same viewing plane. A light source consisting of a triplet of LEDs and a camera with high-speed electron multiplier charge-coupled charge coupled device (EMCCD) imaging sensors is used. The lens system and the switching circuit allow the recording of voltage and calcium parameters at the same time with the same camera. We record transmembrane voltage using a voltage-dependent fluorescent compound (di-8-ANEPPS) while Ca2+ is recorded with fura-4F. By obtaining phase maps and singularities we can characterise activation patterns and the degree of organisation of complex arrhythmias.