Title: CRAP: Semi-supervised Learning via Conditional Rotation Angle Prediction. Abstract: Self-supervised learning (SlfSL), aiming at learning feature representations through ingeniously designed pretext tasks without human annotation, has achieved compelling progress in the past few years. Very recently, SlfSL has also been identified as a promising solution for semi-supervised learning (SemSL) since it offers a new paradigm to utilize unlabeled data. This work further explores this direction by proposing a new framework to seamlessly couple SlfSL with SemSL. Our insight is that the prediction target in SemSL can be modeled as the latent factor in the predictor for the SlfSL target. Marginalizing over the latent factor naturally derives a new formulation which marries the prediction targets of these two learning processes. By implementing this framework through a simple-but-effective SlfSL approach -- rotation angle prediction, we create a new SemSL approach called Conditional Rotation Angle Prediction (CRAP). Specifically, CRAP is featured by adopting a module which predicts the image rotation angle \textbf{conditioned on the candidate image class}. Through experimental evaluation, we show that CRAP achieves superior performance over the other existing ways of combining SlfSL and SemSL. Moreover, the proposed SemSL framework is highly extendable. By augmenting CRAP with a simple SemSL technique and a modification of the rotation angle prediction task, our method has already achieved the state-of-the-art SemSL performance.