This chapter proposes the STECA (STECA – Security Threats, Effects and Criticality Analysis) approach to perform security assessment of Smart Grids. The hereby proposed process describes a way in which to identify vulnerabilities, their related threats, and proposes a risk assessment approach and a path to identify appropriate countermeasures. This process is based on the same principles used for the Failure Mode and Effect Analysis (FMEA)/FMECA process, which is a technique widely used for safety critical analysis and is highly regarded by the majority of international standards [6]. STECA starts from a vulnerability point of view and moves on towards threat analysis and criticality assessment. Following the guidelines defined in [7], the approach is instantiated on a Smart Grid use case, resulting in a set of precise guidelines and a systematic way to perform security assessment including vulnerability evaluation and attack impact analysis.