Cardiac arrhythmias in people display a bewildering variety. This variety arises as a consequence of a small number of basic processes in which oscillations can spontaneously start or stop, or in which conduction can occur abnormally as a consequence of abnormal anatomy, or block in the normal pathways. We describe experimental studies of cardiac activity in heterogeneous tissue culture. The physiological properties of the tissue culture can be modified by using a number of different techniques including the addition of drugs, changing the density, and changing the age in culture. The cells can also be grown in different geometries. Independent of the manipulations, we observe certain types of rhythms including stable pacemakers, stable spirals, bursting rhythms in which there is a spontaneous starting and stopping of the rhythm, and fractionated disorganized rhythms. Investigation of mathematical models of cardiac tissue demonstrates the same types of dynamics as the parameters describing cardiac excitability, cardiac coupling and use-dependent fatigue are modified. This work thus provides a basis for analyzing the rich spontaneous nature of cardiac arrhythmias in people.