  Let $G=(V,E)$ be an undirected graph. We call $D_t \subseteq V$ as a total
dominating set (TDS) of $G$ if each vertex $v \in V$ has a dominator in $D$
other than itself. Here we consider the TDS problem in unit disk graphs, where
the objective is to find a minimum cardinality total dominating set for an
input graph. We prove that the TDS problem is NP-hard in unit disk graphs.
Next, we propose an 8-factor approximation algorithm for the problem. The
running time of the proposed approximation algorithm is $O(n \log k)$, where
$n$ is the number of vertices of the input graph and $k$ is output size. We
also show that TDS problem admits a PTAS in unit disk graphs.
