Current parallel and distributed networks/systems are facing serious threats from network terrorism and crime, which cause huge financial loss and potential life hazard. As attacking tools are becoming more widely available, more easy-to-use, more sophisticated, and more powerful, more efforts have been made in building more effective, more intelligent, and more adaptive defense systems which are of distributed and networked nature. This special issue focuses on issues related to Network and System Security, such as authentication, access control, availability, integrity, privacy, confidentiality, dependability and sustainability of computer networks and systems. We are delighted to present to you four technical papers dealing with cuttingedge research and technology in this field. These papers were selected from either the best papers of the Fourth International Conference on Network and System Security (NSS 2010) or through open submissions. We have received 14 submissions from all around the world and only 4 papers were selected into this special issue based on at least three reviews from international experts. The selection process has been very rigorous and the acceptance rate is around 28.6%. In the first paper entitled “Masquerade Mimicry Attack Detection: A RandomisedApproach”, Tapiador andClark (2011) introduce a class of masquerade mimicry attacks and propose two different detection schemes to thwart such attacks. They use a blind randomisation strategy to build a baseline anomaly detector. Then a Probabilistic Padding Identification (PPI) algorithm based on the KullbackeLeibler divergence is proposed to identify if a sufficiently anomalous attack is present within an apparently normal behavioural pattern. In the second paper entitled “A Pitfall in Fingerprint Biocryptographic Key Generation”, Zhang et al. (2011) present an investigation on analyzing the underlying mechanisms of fingerprint image rotation processing and potential effect on the major features of the rotation transformed fingerprint. It is observed that the information integrity of the original fingerprint image can be significantly compromised by image rotation transformation process, which can cause noticeable singular point change and produce a non-negligible number of fake minutiae. It is found that the quantization and interpolation process can change the fingerprint features significantly without affecting the visual image. This has revealed a pitfall that needs to be avoided in generating biocryptographic key. In the third paper entitled “Hierarchical Attribute-Based Encryption and Scalable User Revocation for Sharing Data in Cloud Servers”, Wang et al. (2011) propose a hierarchical attribute-based encryption scheme (HABE) by combining a hierarchical identity-based encryption (HIBE) system and a cipher text-policy attribute-based encryption (CP-ABE) system, to solve the trust problems in cloud computing. It can provide not only fine-grained access control, but also full delegation and high performance. They also propose a scalable revocation scheme by applying proxy re-encryption (PRE) and lazy re-encryption (LRE) to the HABE scheme, so as to efficiently revoke access rights from users. In the fourth paper entitled “Injecting Purpose and Trust into Data Anonymisation”, Sun et al. (2011) propose a fine level anonymisation scheme with regard to the data requester’s trust and specific application purpose. They firstly prioritize the attributes for anonymisation based on their importance to application purposes. Secondly, they build the projection between the trust value and the degree of data anonymisation, which intends to determine to what extent the data should be anonymised. A decomposition algorithm is developed to find the desired anonymous solution, which ensures the uniqueness and correctness. We sincerely hope that you will enjoy reading these papers and find themuseful.We thank all the international reviewers for their professional services. We deeply thank the Editors Professor Eugene H. Spafford and Professor Dimitris Gritzalis, for providing this opportunity to publish this special issue and their continuous support, encouragement and guidance throughout this publishing project. We also thank Mary Harty for her administrative support.