The GCYCLEFAT project addressed the emerging challenge of predicting fatigue behavior in engi-neering alloys subjected to loading cycles beyond 107, within the Giga-Cycle Fatigue (GCF) regime. In this domain, traditional fatigue models lose reliability, as crack initiation frequently shifts from the sur-face to internal defects, making microstructural features—such as defect type and density—critical to fatigue life prediction. Nevertheless this feature could be considered material dependent. Therefore, sev-eral metallic alloys were investigated, representing distinct manufacturing routes and defect populations: a hot-rolled structural steel (S690), a spring steel (51CrV4), a lightweight cast Al–Si alloy, and an addi-tively manufactured alloy (DED IN625). High-frequency ultrasonic fatigue testing (20 kHz) was the core methodology, supported by low-frequency baseline tests to evaluate frequency effects, modelled numer-ically with thermal–mechanical simulations to capture self-heating during cycling. For structural steel, both uniaxial and multiaxial fatigue tests were conducted, including cruciform specimens and notched geometries, under constant and block loading. The cast aluminium alloy, often used in thin-walled transport structures, was tested using specially designed planar specimens compati-ble with thin-section extraction. For the AMed material, known for high internal porosity, UHCF testing focused on constant amplitude loading and microstructure-fatigue life correlations. The project delivered robust insights into frequency effects, thermal damage modelling, and multiaxi-al fatigue. GCYCLEFAT provided testing methodologies in the giga-cycle fatigue regimes, and en-hanced fatigue design frameworks for long-life structural applications and contributes to safe, cost-effective engineering solutions.