Die Dissertation wurde am 02.03.2012 bei der Technischen Universität München This thesis describes work on two components of the interactive theorem prover Isabelle/HOL that generate proofs and counterexamples for higher-order conjectures by harnessing external first-order reasoners. Our primary contribution is the development of Nitpick, a counterexample generator that builds on a first-order relational model finder based on a Boolean satisfiability (SAT) solver. Nitpick supports (co)inductive predicates and datatypes as well as (co)recursive functions. A novel aspect of this work is the use of a monotonicity inference to prune the search space and to soundly interpret infinite types with finite sets, leading to considerable speed and precision improvements. In a case study, Nitpick was successfully applied to an Isabelle formalization of the C++ memory model. Our second main contribution is the further development of the Sledgehammer proof tool. This tool heuristically selects facts relevant to the conjecture to prove,