Hardware for implementing a Deep Neural Network (DNN) having a convolution layer. A plurality of convolution engines are each operable to perform a convolution operation by applying a filter to a data window, each filter comprising a set of weights for combination with respective data values of a data window, and each of the plurality of convolution engines comprising: multiplication logic operable to combine a weight of a filter with a respective data value of a data window; control logic configured to cause the multiplication logic to combine a weight with a respective data value if the weight is non-zero, and otherwise not cause the multiplication logic to combine that weight with that data value; and accumulation logic configured to accumulate the results of a plurality of combinations performed by the multiplication logic so as to form an output for a respective convolution operation.