This paper presents an accurate and fully analytical model of delay in subthreshold inverters. The model characterizes the direct connection between the input slew and output delay. It is also capable of predicting the signal slew at the inverter output. Delay and slew prediction models are used to compute delay and understand slew propagation in an inverter chain. These models can also provide insight into skew analysis of subthreshold clock networks. Variability analysis across power supply and threshold changes ensure the accuracy of the model.