This thesis is concerned with the inverse problem of determining a unitary connection $A$ on a Hermitian vector bundle $E$ of rank $m$ over a compact Riemannian manifold $(M,g)$ from the Dirichlet-to-Neumann (DN) map $\Lambda A$ of the associated connection Laplacian $dA\ast dA$. The connection is to be determined up to a unitary gauge equivalence equal to the identity at the boundary.

In our first approach to the problem, we restrict our attention to conformally transversally anisotropic (cylindrical) manifolds $M⋐R\times M0$. Our strategy can be described as follows: we construct the special Complex Geometric Optics solutions oscillating in the vertical direction, that concentrate near geodesics and use their density in an integral identity to reduce the problem to a suitable $X$-ray transform on $M0$. The construction is based on our proof of existence of Gaussian Beams on $M0$, which are a family of smooth approximate solutions to $dA\ast dAu=0$ depending on a parameter $\tau \in R$, bounded in $L2$ norm and concentrating in measure along geodesics when $\tau \to \infty$, whereas the small remainder (that makes the solution exact) can be shown to exist by using suitable Carleman estimates.

In the case $m=1$, we prove the recovery of the connection given the injectivity of the $X$-ray transform on $0$ and $1$-forms on $M0$. For $m>1$ and $M0$ simple we reduce the problem to a certain two dimensional $new\; non-abelian\; ray\; transform$.

In our second approach, we assume that the connection $A$ is a $Yang-Mills\; connection$ and no additional assumption on $M$. We construct a global gauge for $A$ (possibly singular at some points) that ties well with the DN map and in which the Yang-Mills equations become elliptic. By using the unique continuation property for elliptic systems and the fact that the singular set is suitably small, we are able to propagate the gauges globally. For the case $m=1$ we are able to reconstruct the connection, whereas for $m>1$ we are forced to make the technical assumption that $(M,g)$ is analytic in order to prove the recovery.

Finally, in both approaches we are using the vital fact that is proved in this work: $\Lambda A$ is a pseudodifferential operator of order $1$ acting on sections of $E|\partial M$, whose full symbol determines the full Taylor expansion of $A$ at the boundary.