This book is designed as an advanced guide to Fourier series. This covers the following topics: Fourier functions and their coefficients, symmetries, odd and even functions, half wave symmetry, fourth wave symmetry, hidden symmetry, problems with symmetry, complex Fourier series, complex frequency spectra, the impulse function. This is including the deduction of many formulas to clarify the concepts and meet all the needs of the students to reach the fundamentals of the Fourier series. Finally, another objective of the book is to reach the interest and motivation of the students by the real nature of the Fourier series.