Onsager derived reciprocal relations for irreversible processes, including heat conduction, electricity conduction, and diffusion, based on microscopic reversibility. He showed that the symmetry of the dissipation function, derived from the general relation S = k log W between entropy and probability, leads to reciprocal relations between different irreversible processes. These relations are valid when the rates of processes are linearly related to the "forces" driving them. The symmetry conditions for microscopic reversibility are equivalent to a variation principle that determines the rates of processes for given displacements. The dissipation function has a statistical significance similar to entropy. External magnetic fields and Coriolis forces break this symmetry, but reciprocal relations involving field reversal are formulated. The paper also discusses the general theory of fluctuations, the regression of fluctuations, and the principle of least dissipation of energy. It shows that the symmetry of the dissipation function leads to reciprocal relations between different processes. The paper concludes with a discussion of non-reversible systems, where the symmetry conditions for microscopic reversibility do not hold, and reciprocal relations between the Nernst and Ettingshausen effects are derived.Onsager derived reciprocal relations for irreversible processes, including heat conduction, electricity conduction, and diffusion, based on microscopic reversibility. He showed that the symmetry of the dissipation function, derived from the general relation S = k log W between entropy and probability, leads to reciprocal relations between different irreversible processes. These relations are valid when the rates of processes are linearly related to the "forces" driving them. The symmetry conditions for microscopic reversibility are equivalent to a variation principle that determines the rates of processes for given displacements. The dissipation function has a statistical significance similar to entropy. External magnetic fields and Coriolis forces break this symmetry, but reciprocal relations involving field reversal are formulated. The paper also discusses the general theory of fluctuations, the regression of fluctuations, and the principle of least dissipation of energy. It shows that the symmetry of the dissipation function leads to reciprocal relations between different processes. The paper concludes with a discussion of non-reversible systems, where the symmetry conditions for microscopic reversibility do not hold, and reciprocal relations between the Nernst and Ettingshausen effects are derived.