The Speech Signal Processing ToolKit (SPTK) is an opensource suite of speech signal processing tools, which has beendeveloped and maintained by the SPTK working group and haswidely contributed to the speech signal processing communitysince 1998. Although SPTK has reached over a hundred thousand downloads, the concepts as well as the features have notyet been widely disseminated. This paper gives an overview ofSPTK and demonstrations to provide a better understanding ofthe toolkit. We have recently developed its differentiable PyTorch version, diffsptk, to adapt to advancements in the deeplearning field. The details of diffsptk are also presented in thispaper. We hope that the toolkit will help developers and researchers working in the field of speech signal processing.