Free Open Source Software have been growing enormously in the field of information technology. Open Source Software (OSS) is a software whose source code is accessible for alteration or enrichment by other programmers. This book gives a detailed analysis of open source software and their fundamentals, and so is meant for the beginners who want to learn and write programs using Open Source Software. It also educates on how to download and instal these open source free software in the system.The topics covered in the book broadly aims to develop familiar Open Source Software (OSS) associated with database, web portal and scientific application development. Software platforms like, Android, MySQL, PHP, Python, PERL, Grid Computing, and Open Source Cloud, and their applications are explained through various examples and programs. The platforms like OSS and Linux are also introduced in the book. Recapitulation given at the end of each chapter enables the readers to take a quick revision of the topics. Numerous examples in the form of programs are given to enable the students to understand the theoretical concepts and their applicative knowledge. The book is an introductory textbook on Open Source Software (OSS) for the undergraduate students of Computer Science Engineering (CSE) and postgraduate students of Computer Application (MCA). Salient Features The procedure for installing software (Linux, Android, PHP, MySQL, Perl, and Python) both in Linux and Windows operating systems are discussed in the book.• Numerous worked out example programs are introduced.• Inclusion of several questions drawn from previous question papers in chapter-end exercises.