Using 3D direct stochastic optical reconstruction microscopy (dSTORM)， we developed novel approaches to quantitatively describe the nanoscale， 3D organization of ryanodine receptors (RyRs) in cardiomyocytes. Complex arrangements of RyR clusters were observed in 3D space， both at the cell surface and within the cell interior， with allocation to dyadic and non-dyadic pools. 3D imaging importantly allowed discernment of clusters overlapping in the z-axis， for which detection was obscured by conventional 2D imaging techniques. Thus， RyR clusters were found to be significantly smaller than previous 2D estimates. Ca release units (CRUs)， i.e. functional groupings of neighbouring RyR clusters， were similarly observed to be smaller than earlier reports. Internal CRUs contained more RyRs in more clusters than CRUs on the cell surface， and yielded longer duration Ca sparks.