Fluorescence optical imaging techniques have revolutionized the field of cardiac electrophysiology and advanced our understanding of complex electrical activities such as arrhythmias. However, traditional monocular optical mapping systems, despite having high spatial resolution, are restricted to a two-dimensional (2D) field of view. Consequently, tracking complex three-dimensional (3D) electrical waves such as during ventricular fibrillation is challenging as the waves rapidly move in and out of the field of view. This problem has been solved by panoramic imaging which uses multiple cameras to measure the electrical activity from the entire epicardial surface. However, the diverse engineering skill set and substantial resource cost required to design and implement this solution have made it largely inaccessible to the biomedical research community at large. To address this barrier to entry, we present an open source toolkit for building panoramic optical mapping systems which includes the 3D printing of perfusion and imaging hardware, as well as software for data processing and analysis. In this paper, we describe the toolkit and demonstrate it on different mammalian hearts: mouse, rat, and rabbit. Reference: Gloschat C, Aras K, Gupta S, Faye NR, Zhang H, Syunyaev RA, Pryamonosov RA, Rogers JM, Kay MW, Efimov IR, RHYTHM: An Open Source Imaging Toolkit for Cardiac Panoramic Optical Mapping. Scientific Reports, 2018 Feb 13; 8(1): 2921. PMID: 29440763. Open source publication link: https://www.nature.com/articles/s41598-018-21333-w. Open source software and hardware are published here: https://github.com/optocardiography/RHYTHM-2.0. Software is written in Matlab. Hardware can be printed on 3D printer using provided STL files.