Solitons are a fascinating and significant phenomenon in various scientific fields. These optical solitons can maintain their shape while propagating through optical media. There are three main types: temporal solitons, spatial solitons, and spatio-temporal solitons. Temporal solitons form through a balance between self-phase modulation and dispersion effects. Spatial solitons result from the interaction between self-focusing and self-diffraction phenomena. Optical media respond to incident beams either locally or nonlocally. In localized responses, the material's optical behavior is confined to the point of interaction, while in nonlocal responses, it is distributed over a spatial range. Optical solitons can form in both local and nonlocal nonlinear media. Local media produce conventional solitons, while nonlocal media generate nonlocal solitons. Nonlocality in optical materials can be classified into four types: local, strongly nonlocal, generally nonlocal, and weakly nonlocal. This book explores the effects of higher-order diffraction and nonlinearity on nonlocal solitons. It also discusses the linear stability analysis and modulation instability of nonlocal solitons.