We implement a least-squares deconvolution (LSD) code to study magnetic fields on cool stars. We first apply our code to high resolution (R ≈ 60,000) optical echelle spectra of several solar-like stars as tests of the code and to measure the mean longitudinal magnetic field on the surface of these stars. Null results or weak fields are detected on the solar-like stars as is expected, demonstrating the functionality of the LSD code. We then apply our LSD code to 6 nights of data on the Classical T Tauri Star BP Tau. A maximum field of 330 ± 80 G is detected from the photospheric lines on BP Tau. Measurements of several emission lines (He I 5876 A, Ca II 8498 A and 8542 A) show the presence of strong magnetic fields in the line formation regions, which are believed to be the base of the accretion footpoints.