This chapter presents the basic concepts in steganography. It shows key concepts in steganography using digital images as media, however many of the concepts presented can be used for other media acquired by sensors, such as sound or videos. The theoretical contributions that make it possible to satisfy the constraints presented generally use information theory, coding theory and image analysis. The embedding capacity, that is, the maximum size of the message that the steganography algorithm is likely to embed in the cover content, is calculated using the Source Coding Theorem by C. Shannon. The chapter explains the different implementation methods in steganography, which mainly differ by the function used to calculate the costs. Side information embedding methods use a reliable measure of undetectability based on the quantization error produced when transforming the image, such as JPEG compression, a geometric or colorimetric transformation followed by a recompression.