This chapter focuses on the history of the concept solidarity, and from this overview tries to find a effective use of it in the twenty-first century. As early as the sixteenth century, French lawyers spoke of solidarité, a word that corresponds to the English ‘joint and several liability’, or what we in common parlance call ‘all for one and one for all’. The concept solidarity starts with the French philosopher, Charles Fourier, in 1808 and becomes a central concept in radical political thought in the nineteenth and twentieth centuries. The provisional rules of the First Workers’ International spoke of ‘solidarity among workers of various trades in every country’. Also, some Catholic philosophers used the word ‘solidarity’ but saw it as an equivalent of ‘charity’. This is a very different use of the term, making it traditional rather than radical. Now we live in a period of a rapidly increasing inequality, catastrophic climate change, a militant world-wide neo-nationalism and ‘alternative facts’ against which solidarity as a common goal through which to create a more just, egalitarian, climate-neutral world is of utmost importance.