In this paper, we present our recent studies on pilot allocation and interference coordination for heterogeneous networks (HetNets) using massive multi-input-multi-output (mMIMO) technology in time division duplex (TDD) mode, where the macro-cell base station (MBS) and overlaid small-cell base stations (SBSs) share the same time-frequency resources, and thus co-tier and cross-tier interferences occur. We investigate how to coordinate such undesirable interference for maximizing the system sum-rate under required constraints. As the first approach, we present an uplink pilot allocation scheme to enhance the downlink system sum-rate by coordinating downlink cross-tier interference to small-cell users (SUs) based on the estimated uplink channel state information. In this approach, we study the optimum pilot allocation against the trade-off between two degradation factors, i.e., uplink pilot overhead and downlink cross-tier interference. As the second approach, we present a dynamic SBS clustering scheme to mitigate dominant co-tier interference among small cells (SCs), where the SBS clustering is carried out based on potential mutual co-tier interference strength between two SCs. We also consider the SBS precoder design in each cluster to further improve the downlink sum rate of SCs under SBS power constraint. Simulation results show that our proposals are useful approaches to enhance the sum rate performance in TDD-mMIMO-HetNets.