In theoretical physics, fine-tuning is a necessary procedure of fudging and very accurate adjusting of the values of the parameters of a theory in order for various physical quantities to be very small. Physicists do not like fine-tuning because the natural values of dimensionless parameters should be of order one. If some parameters must be chosen very exactly in order to reproduce the Universe, even qualitatively, it indicates that there are some new physical mechanisms still waiting to be understood.

The necessity of fine-tuning leads to various problems that do not show that our theories are incorrect, but nevertheless indicate that a piece of the story is missing. For example the cosmological constant problem (why is the cosmological constant so small?); the hierarchy problem; the strong CP problem, and others.