Introduction to Redberry: a computer algebra system designed for tensor manipulation. In this paper we introduce Redberry --- an open source computer algebra system with native support of tensorial expressions. It provides basic computer algebra tools (algebraic manipulations, substitutions, basic simplifications etc.) which are aware of specific features of indexed expressions: contractions of indices, permutational symmetries, multiple index types etc. Redberry supports conventional LaTeX-style input notation for tensorial expressions. The high energy physics package includes tools for Feynman diagrams calculation: Dirac and SU(N) algebra, Levi-Civita simplifications and tools for one-loop calculations in quantum field theory. In the paper we give detailed overview of Redberry features: from basic manipulations with tensors to real Feynman diagrams calculation, accompanied by many examples. Redberry is written in Java 7 and provides convenient Groovy-based user interface inside the high-level general purpose programming language environment. Redberry is available from http://redberry.cc/