Basic question but

$\displaystyle p(\theta | D) = \frac{p(D|\theta)p(\theta)}{p(D)} = \frac{p(D|\theta)p(\theta)}{\int p(D|\theta)p(\theta)}$

Why is $\displaystyle p(D) = \int p(D|\theta)p(\theta)$? I thought it should be the sum? Wouldn't the integral just be 1?

Thankyou