Formal languages have recently come under attention as a powerful tool to describe in a precise and rigorous way mission specifications (i.e., mission objectives and constraints) for robotic systems, and to design planning and control algorithms that provably achieve the specifications. In this paper, we consider Process Algebras as a mission specification language for teams of UAVs. The main advantage of the proposed approach is that Process Algebra specifications are amenable to efficient planning algorithms. In particular, a tree-search algorithm is presented that computes a feasible plan in polynomial time. Branch-and-bound techniques are used to compute an optimal plan if computation time is available. Several mission specification examples are presented, and results from a numerical simulation are discussed.