The special issue on "New Development for Heat Transfer in Solids and Fluid Flow" of the journal "Defect and Diffusion Forum" addresses a number of pertinent issues related to novel analytical and numerical modelling of various problems with respect to heat transfer in solids and fluid flow. Papers presented cover exothermic kinetics in solids, nanofluids and hybrid nanofluids dynamics, natural convection, forced convection, mixed convection, magnetohydrodynamics, biotransport, porous media flow and entropy generation analysis. We anticipate that our special issue will stimulate and help a wide audience of researchers, engineers and educators from various fields of human activity.