Threefolds with one apparent double point

The number of apparent double points of an irreducible projective variety $X$ of dimension $n$ in $\mathbb{P}^{2n+1}$ is the number of secant lines to $X$ passing through a general point of $\mathbb{P}^{2n+1}$. This classical notion dates back to Severi. Smooth threefolds having one apparent double point (shortly OADP threefolds) have been classified in 2004 by Ciliberto, Mella and Russo. In 2011, Ciliberto and Russo have classified normal OADP threefolds such that the so-called fundamental surface is a plane. In this thesis the case in which the fundamental surface is not a plane is considered. A partial classification is given and several examples are presented.