All available sources of data for the specific isochoric heat capacity of technically important fluids and fluid mixtures have been collected and evaluated in this book. Results of critical analyses of the uncertainty of each data source, and comparisons with other sources are presented. All experimental data are compared with available reference fundamental and crossover equations of state. Special interest has been focused on the description of the isochoric heat capacity behaviour along the coexistence curve, critical and supercritical regions. This book also includes a discussion of the thermodynamic background and basic thermodynamic relations connected with isochoric heat capacity as thermodynamic quantity.