The "secret garden of politics", where some win and others lose their candidate selection bids, and why some aspirant candidates are successful while others fail have been enduring puzzles within political science. This book solves this puzzle by proposing and applying a universally applicable multistage approach to discover the relationship between selection rules, selectors’ biases, aspirants’ attributes, and selection outcomes. Rare party and survey data on winning and losing candidates and insider views on what it takes to win a selection contest at multiple selection stages are compared and used to reveal the inner workings of the secret garden. With a primary focus on the British Labour party over several elections, the findings challenge many long-held assumptions about why some aspirant candidate types are successful over others and provides real-world and controversial solutions to addressing women’s and other marginalised groups’ descriptive underrepresentation. As such, it provides a much-needed fresh look at party selection processes and draws new conclusions as to why political underrepresentation occurs and should inform policies to remedy it. This text will be of key interest to scholars and students of gender and ethnicity in politics, political parties and candidate selection, and more broadly to the study of political elites, comparative politics, sociology, labour studies, gender, race, and disability studies, and to practitioners.