1. A method comprising: receiving data relevant to drilling and completion of an oil or gas well, and to reservoir development; and simultaneously calculating well trajectory and drilling and completion decision parameters by using a computer-based model that accounts for an uncertain parameter to optimize an objective function that generates a plan for drilling and completion of one or more oil or gas wells, wherein the objective function optimizes one or more performance metrics that include reservoir performance, well drilling performance, and financial performance, subject to satisfying constraints on the drilling; and wherein the model comprises a Markov decision process-based model and wherein the using of the computer based model comprises solving the equation: L ⁡ ( x ~ K + 1 , u ~ K , ω ~ K ) = ∑ k = 1 K ⁢ l ⁡ ( x k , u k , ω k ) + l K + 1 ⁡ ( x K + 1 ) where {tilde over (x)} K+1 =(x 1 , x 2 ,. .. , x K+1 ) is history of states; X k+1 (x k ,u k )={x k+1 εX|∃ω k εΩ(X k ,u k ) such that x k+1 =ƒ(x k , u k ,ω k )} is the drilling process model; ũ K =(u 1 , u 2 ,. .. , u K ) is history of action variables; and {tilde over (ω)} K =(ω 1 , ω 2 ,. .. , ω K ) is the history of the environment variables.