An additive manufacturing process is monitored, in situ, using optical emission spectroscopy to analyze the composition, phase transformation or manufacturing defects. The system or method may include an analysis of contours of the plasma line intensity, or pre-processing of the plasma spectral line including signal-to-noise ratio analysis, baseline removal, line identification, line de-convolution and fitting. Improvements may additionally involve consideration of plasma parameters such as plasma spectral line intensity, line ratio, plasma temperature and electron density using high-resolution optical emission spectroscopy in both visible and ultraviolet regions. Parameters of the plasma may be determined using an intensity ratio of the ions or atoms emission lines, a FWHM of the line profile for electron density estimation, or a Boltzmann plot for plasma temperature estimation. One or more techniques may be used to monitor when there is a lack of deposition.