Pajek is a program package for Windows (32-bit) designed for the analysis and visualization of large networks with thousands of vertices. It is freely available for non-commercial use at its home page. Developed in November 1996, Pajek aims to support network abstraction through recursive factorization, provide powerful visualization tools, and implement efficient algorithms for large network analysis. It supports ordinary networks, 2-mode networks, and temporal networks. Pajek is based on six data types: network, partition, cluster, vector, permutation, and hierarchy. It allows users to define macros for frequently used operations and adapt to specific user groups. Efficient algorithms for determining main parts in acyclic networks, cores, counting triads, and pattern searching are included. Pajek also contains data analysis procedures such as clustering and block-modeling. It is still under development, with the latest version available for download. Pajek is used in various fields, including social network analysis, chemistry, genealogies, Internet networks, citation networks, and data-mining. It is also used in university courses on network analysis. A course book on exploratory social network analysis with Pajek was co-authored with Wouter de Nooy. Pajek emphasizes automatic network layout generation, including spring embedders, eigenvector-based layouts, layer-based layouts, fish-eye views, and block representations. These algorithms can be modified to include constraints, 3D drawing, and manual editing. Vertex and line properties can be represented using colors, sizes, and shapes. Pajek is implemented in Delphi and runs on Windows 32 operating systems. It supports non-native input formats such as UCINET DL files, chemical MDLMOL and BS, and genealogical GEDCOM. Layouts can be exported in formats like Encapsulated PostScript, Scalable Vector Graphics, VRML, MDLMOL, and Kinemages.Pajek is a program package for Windows (32-bit) designed for the analysis and visualization of large networks with thousands of vertices. It is freely available for non-commercial use at its home page. Developed in November 1996, Pajek aims to support network abstraction through recursive factorization, provide powerful visualization tools, and implement efficient algorithms for large network analysis. It supports ordinary networks, 2-mode networks, and temporal networks. Pajek is based on six data types: network, partition, cluster, vector, permutation, and hierarchy. It allows users to define macros for frequently used operations and adapt to specific user groups. Efficient algorithms for determining main parts in acyclic networks, cores, counting triads, and pattern searching are included. Pajek also contains data analysis procedures such as clustering and block-modeling. It is still under development, with the latest version available for download. Pajek is used in various fields, including social network analysis, chemistry, genealogies, Internet networks, citation networks, and data-mining. It is also used in university courses on network analysis. A course book on exploratory social network analysis with Pajek was co-authored with Wouter de Nooy. Pajek emphasizes automatic network layout generation, including spring embedders, eigenvector-based layouts, layer-based layouts, fish-eye views, and block representations. These algorithms can be modified to include constraints, 3D drawing, and manual editing. Vertex and line properties can be represented using colors, sizes, and shapes. Pajek is implemented in Delphi and runs on Windows 32 operating systems. It supports non-native input formats such as UCINET DL files, chemical MDLMOL and BS, and genealogical GEDCOM. Layouts can be exported in formats like Encapsulated PostScript, Scalable Vector Graphics, VRML, MDLMOL, and Kinemages.