In contrast to more than a thousand extrasolar planet candidates detected over the past couple of decades, the theoretical consideration presents itself many difficulties in the course of planet formation. I will review several of these difficulties and discuss some of their possible solutions. Specifically, I will present (1) how the viscous interaction between gas and solid materials could concentrate the latter fast enough to overcome the constant angular momentum loss experienced by meter-sized objects; (2) the probability of collisional destruction between kilometer-sized planetesimals within a turbulent environment; and (3) the problematic fast inward migration of earth-sized planets in a gaseous protoplanetary disk.