Let $\Omega\subset\mathbb{R}^d$ be a unbounded domain. A positive harmonic function u in $\Omega$ that vanishes on the boundary $\partial\Omega$ is called a Martin function on $\Omega$. In this talk, we will discuss various analytic and geometric aspects of Martin functions, namely how fast they grow at infinity, maximum on a slice, and convexity properties of their level lines. If time permits, we will also present a inverse balayage problem from Potential theory.