Scheduling and resource allocation (SRA) strategies play a crucial role in the emerging 5G systems based on massive MIMO (MaMIMO) transmissions. The set of resources to allocate is increased, adding to the time, frequency, modulation and coding resources, the spatial dimension introduced by MaMIMO. These spatial resources are provided by precoding schemes based on the channel state information knowledge at the transmitter side (CSIT). The joint optimization of all the input variables together with the QoS requirements established by the upper layers results in a complex cross-layer optimization problem. Different metric criteria for SRA algorithms are studied in the literature. MaMIMO SRA is based on CSIT knowledge to select a feasible set of users to allocate the resources of a scheduling interval. This article provides a detailed view of the required mechanisms to understand the SRA process in the emerging 5G systems. To conclude this article, some challenging solutions regarding the utilization of heterogeneous scenarios, MaMIMO transmissions, millimeter wave (mmWave) frequencies, and artificial intelligence are presented.