Expert Randy D. Horsak presents an overview of several Daubert-related issues as they relate to the development and presenting of engineering and scientific evidence by an expert witness in a court of law for a class action lawsuit. Federal and State Courts have a wide spectrum of rulings with respect to the conclusion that Daubert applies or does not apply to expert testimony at the certification stage of a class action lawsuit. In some cases, the court may require a complete, conclusive Daubert inquiry, whereas in other cases the court may require only a limited, focused Daubert inquiry. Importantly, Daubert challenges cut both ways, and may be used by both the Plaintiffs and Defendants to their benefit. Typically, however, Daubert inquiries tend to benefit the Defendants in class action lawsuits. This expert’s commentary is based on actual court cases and other information.