Cellular Networks: Where Are the Mobile Users? -- Distributed Data Structures: A Survey -- On Designing Truthful Mechanisms for Online Scheduling -- On Private Computation in Incomplete Networks -- Traffic Grooming on the Path -- Range Augmentation Problems in Static Ad-Hoc Wireless Networks -- On the Approximability of the L(h,k)-Labelling Problem on Bipartite Graphs (Extended Abstract) -- A Tight Bound for Online Coloring of Disk Graphs -- Divide and Conquer Is Almost Optimal for the Bounded-Hop MST Problem on Random Euclidean Instances -- Distributed Exploration of an Unknown Graph -- Two Absolute Bounds for Distributed Bit Complexity -- Finding Short Right-Hand-on-the-Wall Walks in Graphs -- Space Lower Bounds for Graph Exploration via Reduced Automata -- Communications in Unknown Networks: Preserving the Secret of Topology -- An Improved Algorithm for Adaptive Condition-Based Consensus -- Biangular Circle Formation by Asynchronous Mobile Robots -- Hardness and Approximation Results for Black Hole Search in Arbitrary Graphs -- On Semi-perfect 1-Factorizations -- Free-Riders in Steiner Tree Cost-Sharing Games -- On the Feasibility of Gathering by Autonomous Mobile Robots -- Majority and Unanimity in Synchronous Networks with Ubiquitous Dynamic Faults -- Minimizing the Number of ADMs in SONET Rings with Maximum Throughput -- Optimal Gossiping in Square Meshes in All-Port Mode and with Short Packets -- Geometric Routing Without Geometry.This book constitutes the refereed proceedings of the 12th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2005, held in Mont Saint-Michel, France in May 2005. The 22 revised full papers presented were carefully reviewed and selected from 48 submissions. The papers address issues such as topics in distributed and parallel computing, information dissemination, communication complexity, interconnection networks, high speed networks, wireless networking, mobile computing, optical computing, and related areas.