Isabelle is available inside a flexible system framework allowing for logically safe extensions, which comprise both theories and implementations for code-generating, documenting, and specific support for a variety of formal methods.

Though interactive, Isabelle features efficient automatic reasoning tools, such as a term rewriting engine and a tableaux prover, various decision procedures, and, through the Sledgehammer proof-automation interface, external satisfiability modulo theories (SMT) solvers (including CVC4) and resolution-based automated theorem provers (ATPs), including E, SPASS, and Vampire (the Metis[b] proof method reconstructs resolution proofs generated by these ATPs).

While reflecting the procedure that a human mathematician might apply to proving a result, they are typically hard to read as they do not describe the outcome of these steps.

[citation needed] For example, a declarative proof by contradiction in Isar that the square root of two is not rational can be written as follows.

Isabelle has been used to aid formal methods for the specification, development and verification of software and hardware systems.