This study is motivated by the idea to what extent tourism marketing investment contributes to tourism demand expansion. It searches for better estimation methods that can deal with the inter-temporal and cross-section correlation of the disturbances. The effect of omitting the tourism marketing variable, as evidenced by the drastic change in long and short-run elasticity values for all tourism demand models, has emerged clearly. There is a need for the national tourism institutions to have a clear, consistent, and sustained investment policy in tourism marketing activities with respect to enhanced effectiveness in allocating financial resources.Keywords: Tourism Marketing, 12 Developing Countries, Dynamic Panel, Elasticity Values, Omission of Tourism Marketing Variable.JEL: Z33, C23,O50