A smaller, cost-constrained organization may find that a cloud deployment allows them to take advantage of large-scale infrastructure security measures that they could not otherwise afford. Some of the possible advantages include DDOS (distributed denial of service) protection, forensic image support, logging infrastructure, timely patch and update support, scaling resilience, and perimeter protection (firewalls, intrusion detection and prevention services). The adoption of Cloud computing has been increasing for some time and the maturity of the market is steadily growing; not just in volume, choice and functionality, but also in terms of the ability of suppliers to answer the complex security, regulatory and compliance questions that security oversight functions are now asking. In part this growth has been driven by the continued view that cloud services will deliver cost savings and increased flexibility [12]. Legacy information systems typically form the backbone of the information flow within an organization and are the main vehicle for consolidating information about the business. As a solution to the problems these systems pose such as brittleness, inflexibility, isolation, non-extensibility, lack of openness, _etc._, many organizations are migrating their legacy systems to new environments which allow the information system to be more easily maintained and adaptable to new business requirements [13,14]. The essence of legacy system migration is to move an existing, operational system to a new platform, retaining the functionality of the legacy system while causing as little disruption to the existing operational and business environment as possible [15]. Legacy system migration is a very  
Figure 1: Benefits of Cloud computing. Figure 2: Concerns of Cloud computing. expensive procedure which carries a definite risk of failure. Consequently before any decision to migrate is taken, an intensive study should be undertaken to quantify the risk and benefits and fully justify the redevelopment of the legacy system involved [13,14]. The need for enterprises to migrate their IT systems to profit from a wide set of benefits offered by Cloud environments. It is not surprising that one of the many opportunities facing established companies in today's competitive environment is how best to leverage the cloud as resource, and by extension how to migrate their existing IT environment into a Cloud. Of particular concern to the CIO are two aspects associated with migration, cost and risk [16]. Security consistently raises the most questions as consumers look to move their data and applications to the cloud. Cloud computing does not introduce any security issues that have not already been raised for general IT security. The concern in moving to the cloud is that implementing and enforcing security policies now involves a third party. This loss of control emphasizes the need for transparency from cloud providers [17]. In some cases the cloud will offer a better security posture than an organization could otherwise provide. We want to analyze the different existing approaches in the literature about migration processes to Cloud computing while taking into account the security aspects that have to be also moved to Cloud. There are different initiatives that pretend to show the growing importance of migration processes to modernize legacy systems and advance on business needs and services offered by organizations towards an increasing market and for the future. We want to first analyze the different existing proposals to identify and study the most interesting aspects of migration to cloud and then extract the main advantages and disadvantages that exist and identify gaps, challenges and opportunities to be further investigated. In this study we also focus on security issues considered in migration processes as the security in these open environments is very important and has a high value for organizations which wish to move their applications to the cloud.