The unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEXs), floating head and U-tube HEXs is developed by removing the midplane symmetry (MPS) assumption, which assumes a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the HEX or one TS need be considered. The unified theory can be successfully extended to all common types of HEXs, with arbitrary combinations of TS configurations for two TSs, and with arbitrary material properties and temperature for each component due to less assumptions. The effects of unperforated annual plate, TS flange, gravitational (e.g., dead-weight of tubes, catalyst inside tubes, shell side fluid and tube side fluid) and fluid flow pressure loss, bending stiffness of the tubes, TS in-plane stretch, pressure in TS perforations as well as temperature gradient across TS are also evaluated by the unified theory. Theoretical analysis shows that the existing theories of TS design can be derived from the unified theory as special deductions. The limitations and problems faced in the existing theories and codes are systematically reviewed and solutions to these limitations and problems are also provided by the unified theory. Numerical comparisons indicate that predictions given by the unified theory agree well with finite element analysis (FEA), while ASME results are not accurate or not correct and may lead to unsafety design.