we study the $l^2$-betti numbers of fiber bundles $f\rightarrow e\rightarrow b$ of manifolds. under certain conditions (e.g., when $f$ is simply connected), $b^{(2)}_{\ast}(e)$ can be computed using the twisted $l^2$-betti numbers of $b$. we relate the twisted and untwisted $l^2$-betti numbers of $b$ when $\pi_1(b)$ is locally indicable. as an application, we compute $b^{(2)}_{\ast}(e)$ when $b$ is either a surface or a non-positively curved $3$-manifold. this is a joint work with dawid kielak.