#paper # Abstract Recall that for any Boolean algebra (BA) A, the cellularity of A is c(A) = sup{|X | : X is a pairwise-disjoint subset of A}. A pseudo-tree is a partially ordered set (T, ≤) such that for every t in T , the set {r ∈ T : r ≤ t} is a linear order. The pseudo-tree algebra on T , denoted Treealg(T ), is the subalgebra of P(T ) generated by the cones {r ∈ T : r ≥ t}, for t in T . We characterize the cellularity of pseudo-tree algebras in terms of cardinal functions on the underlying pseudo-trees. For T a pseudo-tree, c(Treealg(T )) is the maximum of four cardinals cT , ιT , ϕT , and µT : roughly, cT measures the “tallness” of the pseudo-tree T ; ιT the “breadth”; ϕT the number of “finite branchings”; and µT the number of places where T “does not branch.” We give examples to demonstrate that all four of these cardinals are needed. # Definitions For any Boolean algebra (BA) $A$, the cellularity of $A$ is $c(A) = \sup{|X | : X \text{ is a pairwise-disjoint subset of }A}$.