This book focuses on object-oriented concurrent computing, which can be considered a model of concurrent programming, and proposes a new programming language, ConcurrentSmalltalk, which is based on object-oriented concurrent computing. The book also shows the efficiency of object-oriented concurrent computing through the design, implementation, and evaluation of ConcurrentSmalltalk. ConcurrentSmalltalk is designed to be upwardly compatible with Smalltalk-80. In the book, the ConcurrentSmalltalk object model is first proposed. Next, issues which arise from maintaining compatibility with Smalltalk-80 are discussed. Finally, the ConcurrentSmalltalk virtual machine which executes the ConcurrentSmalltalk programs is proposed.