Welltest interpretation requires diagnosis of reservoir flow regimes to determine the basic reservoir characteristics. In hydraulically fractured unconventional oil and reservoirs, the reservoir flow regimes may not clearly be revealed on diagnostic plots of transient pressure and its derivative due to extensive wellbore storage effect, fracture characteristics, and also heterogeneity and complexity of reservoir. Thus, use of conventional welltest analysis in interpreting the limited acquired data may fail to provide reliable results, causing erroneous outcomes. To overcome such issues, transient analysis techniques that utilize higher order pressure derivatives integrated with rate transient analysis (RTA) can reduce uncertainties associated with welltest analysis and provide better estimate of the reservoir and hydraulic fractures dynamic parameters. This paper illustrates the reservoir flow regimes in non-fractured and hydraulically fractured unconventional reservoirs, proposes welltesting and interpretation methodologies, and presents field examples in order to show how the hydraulic fracture size and the permeability of the stimulated reservoir volume can estimated for hydraulically fractured unconventional gas reservoirs.