Abstract. The set of all bounded linear idempotent operators on a Banach space X is a poset with the partial order defined by P ≤ Q if P Q = QP = P . Another natural relation on the set of idempotent operators is the orthogonality relation defined by P ⊥ Q ⇔ P Q = QP = 0. We briefly survey known theorems on maps on idempotents preserving order or orthogonality. We discuss some related results and open problems. The connections with physics, geometry, theory of automorphisms, and linear preserver problems will be explained. At the end we will prove a new result concerning bijective maps on idempotent operators preserving comparability.1. Introduction. Throughout this paper X will denote an infinite-dimensional real or complex Banach space and B(X) the algebra of all bounded linear operators on X. By X ′ we denote the dual of X. An operator P ∈ B(X) is called an idempotent operator if P 2 = P . If P is an idempotent, P ∈ {0, I}, then the underlying Banach space X can be decomposed into the direct sum X = Im P ⊕ Ker P , where Im P and Ker P denote the image and the kernel of P , respectively. Clearly, P acts like the identity on Im P , while the restriction of P to Ker P is the zero map. Thus, the matrix representation of P with respect to the above direct sum decomposition of X is