Securing ICS systems, and especially integrated ones, is becoming one of the most urgent issues that disquiets not only all industrial actors but also governments. Very important number of industrial entities and infrastructures are so critical that any non contained cyber attack on these entities can cause huge damage to business, to environment and more gravely to national security and people safety.This thesis studies the integration of ICS with Corporate systems from a security standpoint. Our goal is to study integrated ICS systems security vulnerabilities and suggest models and mechanisms to improve their security and protect them against ceyberattacks. After conducting a study on the vulnerabilities of integrated ICS systems (IICS) and the existing security solutions, we focused on the study of defence in depth technique and its applicability to integrated ICS systems. We defined a new generic segmentation method for IICS, SONICS, which simplifies the segmentation of IICS by focusing only on spects that are really significant for segmentation. We next developed an improved version of SONICS, RIICS (Risk based IICS Segmentation), a segmentation method for IICS systems that fills the SONICS gaps by focusing on risk on top of technical and industrial specifications. To complement the segmentation method, we studied segregation and access control solutions. We proposed a new DTE-based l (Domain Type Enforcement) flow control model for integrated ICS systems.