Molecular clouds are a fundamental ingredient of galaxies: they are the channels that transform the diffuse gas into stars. The detailed process of how they do it is not completely understood. We review the current knowledge of molecular clouds and their substructure from scales ∼ 1 kpc $\sim1~\mbox{kpc}$ down to the filament and core scale. We first review the mechanisms of cloud formation from the warm diffuse interstellar medium down to the cold and dense molecular clouds, the process of molecule formation and the role of the thermal and gravitational instabilities. We also discuss the main physical mechanisms through which clouds gather their mass, and note that all of them may have a role at various stages of the process. In order to understand the dynamics of clouds we then give a critical review of the widely used virial theorem, and its relation to the measurable properties of molecular clouds. Since these properties are the tools we have for understanding the dynamical state of clouds, we critically analyse them. We finally discuss the ubiquitous filamentary structure of molecular clouds and its connection to prestellar cores and star formation.

中文翻译：

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从扩散气体到致密分子云核

分子云是星系的基本组成部分：它们是将扩散气体转化为恒星的通道。他们如何做到这一点的详细过程尚不完全清楚。我们回顾了分子云及其子结构的当前知识，从尺度 ~ 1 kpc $\sim1~\mbox{kpc}$ 到细丝和核心尺度。我们首先回顾了从温暖的漫射星际介质到冷而密的分子云的云形成机制、分子形成过程以及热和重力不稳定性的作用。我们还讨论了云聚集质量的主要物理机制，并注意到所有这些机制都可能在过程的不同阶段发挥作用。为了理解云的动力学，我们对广泛使用的维里定理进行了批判性回顾，及其与分子云可测量特性的关系。由于这些属性是我们了解云动态状态的工具，因此我们对其进行了批判性分析。我们最后讨论了分子云无处不在的丝状结构及其与前星核心和恒星形成的联系。