Plant competition determines the diversity and species abundance of natural communities as well as potential yields in agricultural systems. Understanding the mechanisms of plant competition is instrumental to understanding plant performance in true vegetations. In this review, we will address various components of competition between plant individuals with a specific focus on molecular aspects. As plant—plant interactions during competition are multiple and complex, we will focus here on a restricted set of examples of plant traits that are thought to enhance their performance during competition. To respond to competition by neighbours, plants first need to detect these competitors in a reliable way. We discuss the various ways of molecular detection of competition through light-quality signals, nutrient levels, soluble root exudates and volatile organic compounds emitted by neighbouring plants. Once perceived, these signals are translated into responses such as shade avoidance, root foraging and allelopathy. We integrate the various molecular patterns of signal detection and subsequent plant responses, both above- and below-ground and including their interaction. We outline research strategies towards creating a general, mechanistic understanding of how plants increase their performance during competition.