Disclosed is a method of monitoring operation of programmable logic for a streaming processor, the method comprising: generating a graph representing the programmable logic to be implemented in hardware, the graph comprising nodes and edges connecting nodes in the graph; inserting, on each edge, monitoring hardware to monitor flow of data along the edge. Also disclosed is a method of monitoring operation of programmable logic for a streaming processor, the method comprising: generating a graph representing the programmable logic to be implemented in hardware, the graph comprising nodes and edges connecting the nodes in the graph; inserting, on at least one edge, data-generating hardware arranged to receive data from an upstream node and generate data at known values having the same flow control pattern as the received data for onward transmission to a connected node.