The line by line radiative transfer model (LBLRTM) is an accurate and flexible model for the estimation of the spectral radiance and transmittance over the full spectral range (microwave to ultraviolet), using a first-order perturbation algorithm. It is considered as the basic tool for the creation of retrieval algorithms employed by the ground-based and satellite instruments, while the latest updates in spectroscopic factors are derived from the high-resolution transmission molecular absorption (HITRAN) database. A LBLRTMs is continuously updated and validated against highly accurate spectral measurements. Its errors are related to uncertainties in line parameters and shape. The shape is a Voigt line which is a linear combination of approximating functions for the description of all atmospheric levels. LBLRTML is combined with the continuum MT_CKD (Mlawer, Tobin, Clough, Kneizys, Davies) model which in turn includes the atmospheric constituents of water vapor, carbon dioxide (CO2), molecular oxygen (O2), molecular nitrogen (N2), and ozone (O3), and the molecular extinction process (Rayleigh scattering). A recent version of LBLRTM calculates analytically the Jacobians equations for obtaining meteorological parameters. Also, this model version retrieves the optical parameters of clouds related to scattering and emissivity. The LBLRTM is widely used in radiation and climate applications. It is capable to calculate the absorption degrees of various atmospheric constituents which are utilized afterward from climate and weather prediction models for estimating the broadband solar irradiance and the heating rates. Additionally, the complex radiative transfer models with fast computational time are initiated and trained by the LBRTM, since they are used subsequently on numerical weather prediction (NWP) assimilation systems.