The failures of structures have occurred recently on several occasions both under service and extreme loading conditions. Among these, fatigue is cited as one major mode that especially causes unexpected failures of structures. Most of the present day fatigue assessment approaches in service loadings are generally based on a combination of stress histories, Miner's rule and code provided fatigue curve. In ultimate loading conditions, the stress- intensity based classical fracture mechanics and strain-life curve based low cycle fatigue models are commonly accepted to assess fracture. However, previous researches had highlighted a number of shortcomings of the above approaches. This book, therefore, introduces three recently proposed fatigue models to estimate the life of steel structures under service and ultimate loadings. The book consists of concepts of each fatigue model, mathematical formulations and case studies. This book provides a new theoretical platform for structural design and maintenance communities by contributing convenient, precise and reliable fatigue models to describe real life of steel structures.