A set or sequence $U$ in the natural numbers is defined to be avoidable if ${\bf N}$ can be partitioned into two sets $A$ and $B$ such that no element of $U$ is the sum of two distinct elements of $A$ or of two distinct elements of $B$. In 1980, Hoggatt [5] studied the Tribonacci sequence $T=\{t_n\}$ where $t_1=1$, $t_2=1$, $t_3=2$, and $t_n=t_{n-1}+t_{n-2}+t_{n-3}$ for $n\ge 4$, and showed that it was avoidable. Dumitriu [3] continued this research, investigating Tribonacci sequences with arbitrary initial terms, and achieving partial results. In this paper we give a complete answer to the question of when a generalized Tribonacci sequence is avoidable.