In This Project We Study The Existence Of Some Positive, Periodic Solutions Of Systems OfrnFunctional Differential Equations. In The First Topic We Introduce The Delay Differential EquationsrnFrom The Simple Ordinary Differential Equation. In The Second Topic We Apply A Cone TheoreticrnFixed Point Theorem And Determining Values For ï¬ Obtains Conditions For The Existence OfrnPositive Periodic Solutions Of The System Of Functional Differential Equations.rnIn The Third Topic We See The Existence Of Periodic Solutions For A Single Species Model.rnExponential And Logistic Growth Models Are The Most Common. But In This Topic We Would Like TornStudy A Class Of Differential Equation Models For Single Species That Involve A Time Delay. InrnParticular, We Are Interested To Determining The Existence Of Periodic Solutions In PeriodicrnEquations And In Delayed Lotka-Volterra Type Equations For Single Species.rnLastly We Mainly Concern On The Global Existence Of Periodic Solutions In Delayed Multi-SpeciesrnModels Due To Delays And/Or Periodicity. We Will Present The Global Existence Result Of PeriodicrnSolutions In A Class Of Delayed Autonomous Gause-Type Predator-Prey Systems. Secondly, WernPresent An Existence And Uniqueness Result On Periodic Solutions In A Class Of Delayed PeriodicrnSystems