Many civil claims against financial advisers and portfolio managers fail because they do not meet the high threshold set out by Swiss courts for the allegation and proof of the claimant’s loss. This article critically reviews the extensive case law on the topic. We submit that unauthorized financial transactions are distinct from other defective transactions. Following long-established case law on unauthorized fund transfers, an unauthorized financial transaction does not cause a loss to the client ; it merely fails to discharge the custodian’s obligation to keep safe and deliver the relevant financial assets to the client’s order. When the service provided is otherwise defective, the client bears the burden of proving her loss. Case law distinguishes transaction-based and portfolio-based loss assessment. The former applies when discrete defective transactions can be identified. The claimant must generally prove, to the extent of a preponderance of probability, which alternative investment(s) should have been advised or executed. This high hurdle is mitigated by the defendant’s burden to provide specifics when refuting the loss alleged. On the other hand, when the service is overall defective, the courts apply Art. 42 (2) CO to assess the loss at portfolio-level. We offer some fresh thoughts on transaction- and portfolio-based loss assessment.