This paper investigates the downlink resource allocation problem in Orthogonal Frequency Division Multiple Access (OFDMA) Heterogeneous Networks (HetNets) consisting of macrocells and femtocells sharing the same frequency band. The focus is to devise optimised policies for femtocells' access to the shared spectrum, in terms of femtocell transmissions, in order to maximise femto users sum data rate while ensuring that certain level of quality of service (QoS) for the macro-cell users in the vicinity of femtocells is provided. The optimal solution to this problem is obtained by employing the well-known Dual Lagrangian method and the optimal femtocell transmit power and resource allocation solution is derived in detail. However, the optimal solution introduces high computational complexity and may not be feasible to apply in real-time systems. To this end, we propose a heuristic solution to the problem. The algorithms to implement both optimal and efficient suboptimal schemes in a practical system are also given in detail while their complexity is compared. Simulation results show that our proposed dynamic resource allocation scheme a) ensures the macro users QoS requirements compared to the Reuse-1 scheme, where femtocells are allowed to transmit at full power and bandwidth; b) can maintain femto user data rates at high levels, compared to the Orthogonal Frequency Reuse scheme, where the network bandwidth resources are partially divided amongst macro and femtocells; and c) provides performance close to the optimal solution, while introducing much lower complexity.