In this paper, we present SEAHORN, a software verification framework. The key distinguishing feature of SEAHORN is its modular design that separates the concerns of the syntax of the programming language, its operational semantics, and the verification semantics. SEAHORN encompasses several novelties: it (a) encodes verification conditions using an efficient yet precise inter-procedural technique, (b) provides flexibility in the verification semantics to allow different levels of abstraction, (c) uses Horn-clauses as an intermediate language to represent verification conditions which simplifies interfacing with multiple verification tools based on Horn-clauses, and (d) leverages the state-of-the-art in software model checking and abstract interpretation for verification. SEAHORN provides users with a powerful verification tool and researchers with an extensible and customizable framework for experimenting with new software verification techniques.