Fisher's Estimator Inequality

Given $T$ an Unbiased Estimator of $\theta$ so that $\left\langle{T}\right\rangle{}=\theta$. Then

$$\mathop{\rm var}\nolimits (T) \geq {1\over N\int_{-\infty}^\infty \left[{\partial(\ln f)\over\partial\theta}\right]^2 f\,dx},$$

where $\mathop{\rm var}\nolimits$ is the Variance.