In this paper we revisit the theory of negative capacitance, in a (i) standalone ferroelectric, (ii) ferroelectric-dielectric, and (iii) ferroelectric-semiconductor series combination, and show that it is important to minimize the total Gibbs free energy of the combined system (and not just the free energy of the ferroelectric) to obtain the correct states. The theory is explained both analytically and using numerical simulation, for ferroelectric materials with first order and second order phase transitions. The exact conditions for different regimes of operation in terms of hysteresis and gain are derived for ferroelectric-dielectric combination. Finally the ferroelectric-semiconductor series combination is analyzed to gain insights into the possibility of realization of steep slope transistors in a hysteresis free manner.